A project I intend to get working is to use a one-wire network attached to the USB port, using a DS9490 as an interface. Details of this project, including the code (written in ruby) is available at http://github.com/kim-toms/thermostat/tree/master
GPIO for the one-wire interface is available from http://hobby-boards.com/
The one-wire network is comparatively slow, so it depends on the throughput you need. I'm going to use it to control a furnace and monitor a few thermometers.
I currently have the code working on a PC under ubuntu, the sheevaplug will let me turn that off, and use less electricity.