technically NetStumbler is not a passive wifi detection client.
You still "touch" base with the AP's by sending out "beacon" packets.
One other software client that I know of that does not "touch" anything, but rather it listens, is Kismet it sends out no querying packets of any sort.
And edit to some of the posts I have seen regarding MAC filtering: MAC filtering does not encrypt packets therefore you can collect data packets within the protected network, and spoof 1 of the MAC addys that are stamped on the packets. I use a program called SMAC.