iOS 5 is compatible with the iPhone 3GS, iPhone 4, iPhone 4S, iPod touch (third and fourth generation), and the iPad and iPad 2 but not all features will be available for all devices. iOS 5 will unify all compatible devices to the same version of the OS for the first time since iOS 3.1. More information on iOS 5's features can be found on the Apple website.
In addition to iOS 5, Apple also officially announced its iCloud service, a cloud storage, syncing and backup service that's tightly integrated into the new version of iOS and OS X Lion. iCloud replaces MobileMe (which is going away in June 2012) and automatically syncs music, documents, and photos between all your devices.
Sadly, Apple didn't announce a replacement for its MobileMe Gallery, hastening my migration Picasa Web albums.