I too am having the same problem with the iPhone 4, while my high quality BT headset worked flawlessly with my 3G. Callers say I sound like I'm underwater, and they sound marginally worse to me. The reception seems worse as well. As a truck driver, both issues are quite serious. WTF Apple?!
Same problem here. Plantronics 855 and Jabra 8400 were and are perfect on 3GS and 3G and everyone complains Im muddy or in a bucket on the iPhone 4.
I noticed it's a little better if I make or answer the call from the Bluetooth. If I transfer from wired or the phone to Bluetooth it is horrible.
Others are having this trouble too, there is a thread on the apple support website.
Interestingly, my expensive BoomO headset, a wired, high end noise canceling headset will NOT work with the iPhone 4. It sounds like there are windshield wipers on to the listener. The BoomO support said they know about it and they are coming out with an amplified cord for the boom. They said apple changed the circuitry on the headset jack. My old cheap Plantronics wired headset works great.
I hope they get this fixed soon! No Bluetooth is the pits.