I've previously mentioned the Rasberry Pi, the cool credit-card sized Linux computer that is very inexpensive, and can be easily attached to a monitor and keyboard.
Well, I came across this interesting posting at InfoSec Institute resources on using the Rasberry Pi as a Pentest system.
It looks pretty interesting. There are already setup distributions you can download to a SD card that has all the Linux tools you'd need already setup. The author shows 3 different distributions available.