The big weakness of the iPhone when it comes to gaming has been the lack of a social platform. Facebook games are inherently social and so they spread far faster than iPhone games do, and they consequently make a lot more money for developers.
Apple hopes to catch up on that front with Game Center, a social platform for the iPhone, iPad, and iPod Touch. The platform, announced today, and due to be released with version 4.0 of the iPhone OS due out this summer
The Real shame here is that Apple are not using existing games on Facebook such as Farmville because of the lack of support with Flash, I am sure Playfish, Zynga etc will create their titles for the iPad / iPhone in the native language, but this still seems a shame that as a game developer you have to start a fresh and also grow your userbase again.
Zynga = 244,783,375 Facebook users.