Important to note:
if you are a developer, if you download the beta of iPhoneOS 3.0, if you install it on your phone, you cannot go back to an earlier version of the OS!
This means you’d be stuck with 3.0 beta on your phone until the final 3.0 release comes out sometime this summer.
Also, if you download the 3.0 SDK, you can’t submit anything you develop with it to the app store until after the final 3.0 release this summer.
Important gotchas to keep in mind.
That said, I doubt I’ll be able to resist the temptation to try it out.