OK, I just ordered this one and it was inexpensive and rated highly on amazon.com, but I bought it directly through the seller's web site because the shipping costs were a little cheaper:
http://www.buygpsnow.com/HAiCOM-Custom-Car-Mount-for-Apple-iPhone-3G.aspx
9.95 + 2.50 s/h
I will report back when I get it and tell you if I like it or not.