I'm having similar problem with my iPhone 4 and bluetooth headsets. I have both the Motorola H350 and H500 headsets ... older models, but both worked great with previous iPhone 3G and other cell phones. Though I could sometimes hold up to 10 ft of range while on hold for someone, I'd have to be within a couple feet when I was talking or they couldn't understand me ... I'd become muffled and broken up. I had no problem hearing the person I was talking to. But for the past few weeks my iPhone 4 periodically simply won't connect to either headsets no matter what I do (reboot, remove headsets from iPhone, shut off bluetooth then reboot and then turn it back on again, re-pair the iPhone and headsets, etc). Occassionally, it would seem as though it paired again, but wouldn't actually connect (just got the Apple hourglass beside the headset name).
I was reading a post over at
http://www.zdnet.com/blog/apple/add-bluetooth-to-the-iphone-4-list-of-woe-update d-2x/7588 where the author got a replacement iPhone 4 and bluetooth work fine now (origial post was from July 2010, but there have been 2 updates to the article since then). Has anyone else tried getting a replacement iPhone 4? Are people having the problem with brand a new iPhone 4?