As Wayne indicated, I've just recently started using the MOTU Traveler (firewire) at shows, with really good results so far. Running multitrack was a snap. I think the pre-amps are a bit weak; but the unit was so easy to set up and use
I've seen a couple of people at shows using other USB/FIrewire interfaces. Both had 12" Powerbooks, one using a MiniMe via USB; and the other using what looked like an M-Audio Firewire 410.