I have Knoppix 3.8.2 live cd, with a cisco aironet wireless card.
It seems airsnort won't pickup networks unless I am running Kismet first. I manually put the card in monitor mode before I launch any applications.
iwconfig ath0 mode monitor
and make sure that it is in monitor mode by looking at iwconfig ath0.
If, however, I launch kismet, then airsnort, it will pick networks and start looking at packets. Not sure what else Kismet may be doing to the card to make it work.