I got my mini-usb to 3.5 adapter from eBay for $6 shipped (with car charger):
http://cgi.ebay.com/ws/eBayISAPI.dll...MEWN:IT&ih=002
Works well - I get stereo. BUT - some people have reported that these aftermarket adapters can deform the mini-USB port, so I try not to use it all that often. I don't know which adapters would do that and which ones wouldn't (I'm sure there's more than one factory in China making these things). Seidio also sells an adapter, and their stuff is usually of good quality, but it's also much more expensive.