I had the same exact problem with my first gen MacBook Pro. It was either a problem with an antenna or the logic board and I wasn't about to shell out 300 bucks for a 4 year old machine to figure out the problem. I went with this USB network adapter: http://eshop.macsales.com/item/Edimax/EW7718UN/
It works pretty well although the software isn't that great. The only ExpressCard wireless adapter I came across while looking that I know for sure works for Macs is one by QuickerTek which runs for about 150 bucks. Again, no way i'm paying that much so I went with a USB adapter.