AdMod serves ads for more than 9000 mobile web pages and 3000 applications, all over the world. They use this to collect data on handsets and operating systems being used to access these web pages and applications. AdMob serves more than 10 billion ad requests per month. As always, these figures are not in any way to be trusted at face value; they indicate trends, not absolutes.
But boy, are those trends clear. It is the new platforms who are gaining market share at the cost of the older platforms out there. Between February 2009 and August 2009, iPhoneOS, Android, and webOS all gained significantly in share. The iPhone stands out because of its massive 40% share of the market (still growing!), and the webOS stands out because despite its short lifespan so far, it already gained 4% of a highly competitive market, even though it's only available in North America. It is important to note that the iPod Touch was not included in these figures, as it does not fall within the definition of a smartphone used by AdMob.
Even though these figures do not present any absolutes, it's quite clear that the 'new' mobile platforms, designed from the get-go with the web in mind, are the ones that are growing rapidly. The older platforms, all from pretty much a pre-mobile web era, are doing badly and are losing market share.
It's also clear that the iPhone is pretty much the king here, which really shouldn't surprise anyone who hasn't been living under a rock the past two years. Luckily though, the iPhone doesn't strangle the competition, and new players are still capable of grabbing parts of the market.