Yesterday night, Google announced an agreement to acquire AdMob – a Silicon Valley based company that serves display and in-app ads on cellphones – for $750 million. AdMob is one of the largest ad serving network on cellphones and is widely used for ad-supported applications on the iPhone as well as other smartphones.

The move is well-timed for Google, which has stepped into the smartphone arena in a big way with its own OS platform. The timing could not have been better with Android taking off with a couple of high profile devices and many more expected next year.

A huge chunk of Google’s revenues come from ads served within online search results and AdMob’s acquisition would help it take it a step further to the so-called ‘fourth screen’. With apps for smartphone becoming much more popular than web surfing on smartphones itself, it was all but natural for Google to acquire a company like AdMob.

However, I’d like to point out that the deal is much bigger than Android itself. AdMob’s platform is used in majority of adcertising-driven iPhone apps and I won’t be surprised if iPhone would contribute more to Google’s revenues from this deal than all Android phones put together.