Just thought I'd report in... I too am having this issue on an intermittent basis. It is not consistent, but occasionally when using Safari or the iPod, I can't get the UI to rotate. Then, other times, it'll work just fine (right now it is fine). Unlike others here I haven't really noticed any relationship to sleeping at all, but I put it to sleep all the time (every time I put it in my pocket) so I'd really never know. Usually, I can eventually get it to rotate without being forced to turn the phone off and then on again. Sometimes I need to exit out of Safari or the iPod app, but then it'll usually start behaving.
I'm currently using a white 16GB iPhone on 2.0.1. The update doesn't seem to have made any difference. Still happens occasionally.
My issue isn't too bad and there is no Apple store within a 4.5hr drive of here so I'm going to live with it for now and hope for a future firmware fix (besides, I'd never get it to actually misbehave for a Genius in person).