http://www.linux-forensics.com/downloads.html
Grab the Penguin Sleuth Kit Bootable CD.
It's knoppix.. so everything will load OK.
Hit a root shell from the menu.
Type 'airsnort' and knock yourself out.
Can't save captures unless you mount a floppy or usb key or something, but this is the easiest way I've found to just fire it up and take a look around. I have a laptop running in my apartment and have a handful of frames from neighbors. Just small keepalive stuff.. I'll never get enough from them to crack one.. which is unfortunate. But tonight I'll turn on my AP and start moving tons of stuff and see if it gets my key.
One weird thing about airsnort I've discovered.. it gets some of the names wrong. For instance on two seperate bootable cdrom distros, it named the wifi connection 'eth0'.. so in the airsnort menu you have to nuke 'wlan0' and put in 'eth0', or it won't find it. Also the drivers were reversed. My prism2 linksys card wouldn't work with the 'prism2' setting.. but came right up on 'Orinoco'.. haven't tried my orinoco gold card yet.
There's lots of other goodies on the cdrom, too.. check out the intro
http://www.linux-forensics.com/forensics/pensleuth.html