... It took me some time to figure it out, but in my experience, OS 3.0 will use the top button to hang up when you use the iPhone as a handset. Prior OS versions let you lock the phone, and only hang up when you hit the scary red button.
The feature isn't without its inconsistencies. If you use the iPhone with a headset or as a speaker phone, it still behaves as prior versions, locking the phone rather than hanging up when the top button is pressed.
My opinion: it will take some getting used to, and may even be useful as a quick hang up when its a handset (i.e. when you're done with a call, just hit the top button to hang up while its next to your ear.) However, my opinion is that the mere inconsistency of this potentially unwanted and unexpected behavior is a serious minus. If you have a conference call or call-waiting, I don't know what to expect.
Unlabeled and obscure features were a thing other smartphones do with their plastic buttons, not the iPhone. It should be left to the clearly labeled red button. I expect it to revert to the old behavior with 3.0.1.One of the few downsides of the iPhone as a phone is that it's incredibly easy to accidentally disconnect a call, especially when you're using an earset or the speakerphone. The red disconnect screen button is very sensitive.
So I was delighted when I learned the power-off button would blank the screen when I was on a phone call. I use it all the time.
Since I mostly use my earset I wasn't aware the behavior had changed. If you use an earset, it still works properly. Otherwise, it's a problem.
PS. Macintouch is a great place to learn about bugs and issues with OS 3 and with the 3GS. I can confirm my 3G's battery life has fallen sharply, but that might because I've enabled Push notification for Beejive. I haven't noticed the Wifi and network issues others report, but I usually connect to an Apple Airport.