A Nano version would be rubbish, as the main attraction of the iPhone is the decent screen size. Any smaller and it'd be a waste of time.
I believe there's an annual keynote speech planned for June anyway, so expect it to be announced then with a release in July, and regular stock to be around November time if it's popularity is anything like the 3G. You won't be breaking an existing 18month contract, they'll simply sign you up for another 18 months and charge you an extortionate upgrade fee if you only got the 3G within the last 6 months. Well, that's my guess anyway, but tbh if you've got a 3G I can't really see what the iPhone 3 will have that would entice you to upgrade anyway: Certainly the 3G didn't do that much over the 1G, hence I've still got my trusty 8GB one.
The 3.0 firmware will be along in a few months, yes.