While testing out the Jawbone Icon wireless Bluetooth headset recently, the parties I called complained of diminished sound quality on their end. They talked about overly compressed muffled sound, with decreased dynamic range. Over at the Jawbone end-user forums several threads have been discussing exactly that. Plantronics forums are reporting issues as well.
Informal testing here at TUAW central with a couple of Jawbone units on both an iPhone 3GS as well as the iPhone 4 showed that the 3GS displayed none of the problems that the iPhone 4 is demonstrating.