Apple just previewed OS 4.0. Two exciting things stand out for me:
Multi-tasking: I've had an iPhone for almost two years now, and I can't say I've missed multi-tasking. It really hasn't been an issue (but then I listen to audio through the iPod, not Pandora, and don't use Skype, so maybe I am an oddity)
BUT within a day of having the iPad I realized that I really wanted multi-tasking, if nothing else so I could leave the IM app running.
iBooks: they are bringing iBooks to the iPhone, and they talked about "Platforms." iBooks will be on the Mac (and PC) pretty soon.
The BAD news: OS 4 won't be on the iPad until the fall.