Wow, I was just reading about the SheevaPlug this morning. I was thinking about buying one to play with, and perhaps to make a portable LAMP development server (plus, it'd make a cheap little SVN box as well).
Yeah! Wow! That thing is so cool! I dreamed about a device like that years ago. I thought it would be ideal for installing surreptitiously at my school, in order to seed some torrents.
One of the users on their forum said that if you crack it open, there's still a bunch of empty space. Even though it would raise the cost a little bit, I'd like to see it add wifi, bluetooth, powerline networking (so it could act as a router between powerline and ethernet) and X10/zwave compatibility. Those things wouldn't add more than $20-50, and it would make the SheevaPlug a kind of hacker's Swiss army knife.
Yeah, its weird because my SheevaPlug just arrived in the mail 2 days ago and I've been thinking about what to do with it. My original thought was to couple it with my arduino board and see if I can do anything useful with that. On second though, a cheap network storage device and auto-backup system also sounds useful.
If you want a cheap SheevaPlug-based network storage device, you should look at the Pogoplug at www.pogoplug.com
Such device can be great to ship your web products to customers. For example you can create CRM system or web gallery and ship it on SheevaPlug. So client will be able to plug it in his local network and have it working without any complex setup.