At Woot.com, they are selling a Eee 900 netbook for $169 (today only, of course).
It's a limited computer, of course, but that's not the point. What makes it wonderful is that it's disposable. We use these in pentesting, leaving it behind attached to a wired network, or wifi scanning. It has an Atheros WiFi chip, which is the best under Linux for Wifi pentesting. The great thing about it is that if it's destroyed, lost, or stolen, you are out just $170. What's also cool is that you can boot from SD cards with different versions of Linux (e.g. Backtrack), for a complete set of pentest tools.
Another option, btw, is the Acer Aspire One AO751h. I saw one at Costco for $329. It also has an Atheros WiFi for pentesting. However, that unit is 11.6 inches with a full sized keyboard. I've sat in a cramped car pentesting with the Eee 900 -- it would've been much easier with that Acer unit.
I got my mom a Eee 1000he (she travels a lot) for $300. Unfortunately, she doesn't get the idea of "disposable" computer and takes care of it like it's something valuable. She keeps it in the case, even with the slip of cloth inside between the screen and keyboard. She wipes off fingerprints. In my mind, she should treat it more like I treat my MacBook Air, which has picked up numerous dents and scratches since I got it 10 months ago.
A recent episode of the series "Burn Notice" showed the main character leaving behind a netbook monitoring a network. It looked like a product placement and was not material to the plot, but it shows the sort of thing I do with pentesting.