I pulled down the map light cover and added some weather stripping foam (3/4 inch) around the backside of the mic this weekend. Friends that I called said it helped a little but not greatly.
One person originally said it helped a lot but a minute later changed their mind and said it was just as bad as before. They said it was worse (in the middle of my town) when in 3G than when I was driving in the middle west Texas on an EDGE system. One person described it was "... talking in a wind tunnel with bumble bees". Which makes me believe that some, if not all, of the voice quality issue is dependent on the quality of the cell phone carriers signal.
Next step is to stuff some "egg-crate" foam between the headliner and the roof.
I also hope to see if there is a difference between a GSM (AT&T) phone and a Verizon (CDMA) phone in my area.
I have an iPhone 3GS on AT&T (GSM) in a place that barely has 3G towers. So if your familiar with AT&Ts advertising, I'm in that last 1 percent of the 95 percent of America covered by 3G. I wish AT&T would spend more money upgrading their 850 Mhz UMTS system and less on advertising.