Editor’s note: The following guest post is by Krishna Subramanian, co-founder of mobile ad exchange Mobclix.

Mobile search is still one of the big unclaimed prizes on the mobile web. Everyone from Google and Yahoo to Apple is going after it, but Microsoft’s Bing may stealthily become the king of the castle by aggressively promoting Bing through mobile apps. Let’s look at each player’s mobile search strategy.

Apple: In The Driver’s Seat

During the Apple keynote in April, Steve Jobs announced the new iPhone 4.0, iAd and a few other features even he didn’t seem too excited about. Out of the many mediocre features, Mr. Jobs happened to squeeze in a declaration that, “ On mobile, search hasn’t happened. People aren’t searching on their phones.” During the keynote at WWDC this month, Mr. Jobs declared that iPhone 4 users would have the opportunity to select their search engine from among Google, Yahoo, and Bing. Out of the three, Bing got a special endorsement from Mr. Jobs.

Is Mr. Jobs trying to blindside the other players in this space by making them think he is not concerned about search? I’m sure all of the search traffic he is sending to Google is driving him nuts. Meanwhile, Google has happily—and quite beautifully—optimized their search results page on the iPhone to make it extremely convenient for local searches by incorporating phone numbers, maps and more within the Safari window.

Remember the days you would dial 411 or, even more recently, send an SMS to GOOG for information about local businesses or venues while you are on-the-go? Does anyone do that anymore? I’m sure people love paying $1.75 to find out the name of the local pizza shop. By the time you dial 411 and struggle through the automated voice menu, you could have pulled up addresses, phone numbers and reviews to the five nearest pizza places and be one click away from an interactive map.

Apple brought the traffic to mobile search, but why not make money off it? Google, Yahoo! and Microsoft all see the value of controlling search across mobile devices—not just the iPhone. Similar to the web, these three goliaths want to be the starting point for every consumer query. All three have launched iPhone specific apps with slightly different flavors as they try to first win the hearts of the iPhone user.

Yahoo! Doesn’t Know What It Wants To Be

Yahoo! is all over the place in the App Store. It has two iPhone apps in the iTunes App Store. Within the reference category, the Yahoo! Search app is ranked at #30 with 658 comments. For the most part it includes many of the same core features that the other search apps offer.

To get more mindshare from users, Yahoo! has sprinkled many other apps in various categories with the Yahoo! Finance and Yahoo! Shopping and Yahoo! Entertainment apps and a pretty successful Yahoo! News app (#47 with close to 50,000 comments). This attempt to build interest just dilutes Yahoo!’s audience across multiple apps—which, if combined together, could have a significant impact.

It’s Google’s Game To Lose

As the default search engine plugged into mobile Safari on the iPhone, Google has always had an advantage driving mindshare to its mobile apps. It was the first one to use voice activated search and has steadily built out its host of features since making it easier to access core Google products like Gmail. The Google Mobile app is currently ranked first in the reference category for iPhone apps with more than 2,000 comments.

Hello, Bing!

Far behind when the gates opened, the Bing team is pushing out new features as fast as possible, trying to draw from Google what works best. Interestingly enough, the results (even on a local level) are quite different from the very accurate Google search results. The Bing search app received a nice endorsement from Mr. Jobs at the WWDC keynote, so lofty expectations are already set. It is currently ranked No. 2 in Reference.

Microsoft released an updated Bing app last week with a few new notable features: Visual Scanning (very similar to the Red Laser iPhone app eBay acquired earlier this month) and tapping into social graphs through Facebook and Twitter status updates. The entertainment angle is allowing Bing to create a unique niche that ties back to search. It also redesigned its mobile browser search to make it more of an app-like experience.

Advertising as a Distribution Channel

The biggest hurdle is getting these app installed on as many devices as possible, but thanks to all the apps in the App Store there is an abundance of ad inventory available for marketers. App developers, if you don’t already love these big boys, you should. They have been spending significant amounts of money (think six figures-plus) desperately trying to get in front of as many of your users as possible, which translates into more money in your pocket.

All of the search giants use in-app marketing to push their own apps. Yahoo! and Google have done a great job avoiding creative saturation by building out a wide array of messages, colors, languages and landing pages, as well as making use of geo-targeting.

Boom! Bing Changes the Game

As the Bing team continues to spend more money on advertising, they recently changed the game, significantly crushing Google in the app rankings. How? Easy…

Attaching yourself to successful apps with consumer brand power is a sure fire way to rise to the top.“We absolutely market our applications on the iPhone, I don’t think of it as unique to anything else. It is like promoting on the toolbar,” Yusuf Mehdi, Senior Vice President, Bing recently told TechCrunch co-editor Erick Schonfeld. “Yes, it has been effective.”

For example, Bing took the Top 100s by Year app that allows users to stream songs by decade for $1.99, rebranded it to Top 100s by Year by Bing, made it free and inserted advertising to drive users to download the Bing Search App.

The Top 100s by Bing app instantly surged to the top of the App Store and remained in the top five for weeks. It remains at the top of the music category hovering near favorites like Pandora and Shazam. And, as I mentioned, the Bing search app is currently No. 2 in Reference, and in the top 100 free iPhone apps overall.

Ads within the Top 100s app that drive users to download the Bing app:

Fresh off the success of leveraging sponsored apps to drive downloads to the Bing app, Microsoft has recently reached out to a new audience segment by sponsoring the ESPN World Cup Trivia app, which is ranked No. 6 in the sports category.

Rather than taking the viral nature of the Top 100’s music app, Bing and ESPN are also running display inventory to drive additional traffic to the World Cup Trivia app.

So where does this leave the Bing search app? How about in the top 100 of all free apps and, more importantly, at one point it even squeezed Google’s app out of the top 50 free apps. Bing has taken a simple concept, executed and proved the value of the model by consistently keeping their brand top of mind across top apps in different categories.

Mobile search is here, whether you want to believe it or not. Take Apple’s recent acquisition of Siri for a small sum of around $200 million to $250 million. It will be pretty easy to use that as the nucleus for an Apple-owned local search product for mobile. Not to mention the valuation is around the same price tag as what it paid for Quattro Wireless.

As the market grows, Yahoo!, Google, Bing and Apple will become more cut throat. Don’t expect to see Bing ads on Google Mobile Ads or Apple featuring Yahoo!. Likewise, we may not see as many new Google and Bing apps in the App Store in the near future. But they will keep pushing forward as much as they can. After all, they probably don’t have much time before Mr. Jobs begins to think differently about mobile search.