There is a project out there, called RaspberryPwn, that is supposed to be a pentesting Linux distro for the RasPi. It's easy enough to put together your own pentesting tools, especially with something like Arch Linux, but it's still interesting.
Raspberry Pi is not the only platform I have in mind, either. Gooseberry, APC, etc. are similar, each with different specs.
I imagine one could leave a scan running over the course of several days, or weeks, running slow enough to not trigger an IDS, and pick it up later. Some of them are cheap enough that it would be of little concern if you were unable to recover it for some reason (RasPi is only $35).
Anyway, just an idea I had rolling around. Let me know what you think.