UPDATE (06/26/2009): In the meantime, another, major problem has been found out: the encoding lag. See the just-added UPDATE section in the original article.
Originally written: Unfortunately, there are two major problems with the stereo Bluetooth (A2DP) support of iPhone OS 3.
1.) Many of you have already posted the previous/next remote controls not working. This, unfortunately, is a fundamental problem with the AVRCP implementation of OS3. Hope they’ll fix it in a next update.
2.) I’ve found a very hard-to-track and really annoying problem. A lot of (mostly older; that is, older than about two years) A2DP headphone models don’t work acceptably with the iPhone 3G while listening to / watching streamed contents (incl. YouTube) via Wi-Fi. (Local playback and cellular [GPRS etc.] streaming works.)
Please check out http://www.iphonelife.com/blog/87/full-compliance-test-report-stereo-bluetooth-sound-a2dp-iphone-os-3 for more info on this issue (particularly the last two updates posted at 06/24/2009 23.47 CET and 06/25/2009 13:47CET). The, in this respect, useless headphones also include the Plantronics Pulsar 590 (an, otherwise, excellent model).
UPDATE (10/17/20009): iPhone OS 3.1.2 Bluetooth audio update, concerning both the 3G and the 3G S, posted HERE.