I just got NetStumbler to work with my Buffalo CBG54 card with the Broadcom G54 chipset under WinXPproSP1. At first it would notice the card (under the Device menu), but couldn't use it. I'm not sure all these steps were necessary, but what finally made it work was to quit NS, uncheck "Use Windows to configure my wireless", and "disable" the wireless network connection in Network Connections. Then when I started NS, it began working right off. Except the "noise" figure is always -100.
I went "war hiking", and when I got to the edge of my own AP coverage area it momentarily lost the signal it had been tracking, but it "found" a new "peer-to-peer" connection that seemed to be limited to 2Mb and never displayed a name or ID. The "first" time on that connection exactly matched the time when Windows noticed the peer link. For awhile NS was tracking both connections simultaneously, but once I got back closer to the AP it quit tracking the "new" anonymous link.
To get beyond the 2Mb speed of the "peer" connection, I re-checked "Use Windows to configure my wireless". It almost immediately "found" my AP by name and went back to 54Mb. Now I can start NS and it begins polling and playing its piano and reporting signal strength without disrupting my 54Mb link. Wonder if that will survive a reboot...