The D-Link new 802.11b USB adapter had trouble connecting to a Linksys WAP11 set on repeating mode.
I was setting up WiFi at a house and repeating the signal to this remote area about 2000 feet away.
My friends Centrino connected fine, and so did my Lucent Silver WaveLan card, but the USB D-Link was dropping in and out.
I suspect that USB devices are limited on amp draw? And therefor aren't as powerful as the PCMCIA or PC cards?