For an idea of how big a marketplace this is, check out AdMob's own numbers; they already serve ads on mobile Web sites on the Android platform, receiving over 27 million requests in December 2008 alone. On the other hand, AdMob is already serving ads for 450 iPhone applications.
AdMob has recently raised over 15 million dollars in funding; it looks like they're putting it to good use. With new Android devices coming in 2009, folks at AdMob are hoping that the Android platform will take off. "We are already seeing strong interest in developing applications for Android-based devices, similar to what we saw with the iPhone last summer, and are very excited to take the lead in helping developers monetize their applications on this fast-growing device platform," said Ali Diab, Vice President of Product Management for AdMob.