What are some of the more compelling use cases for plug computers?
There are quite a few people (including me) using them with arduinos. Currently it's only being a temperature sensor. I run a MQTT Server on the sheevaplug and use php scripts to get data from the arduino and send messages into the server for receipt by a few clients. I also monitor the sheevaplug usage via MQTT and post it all to pachube. 

My plug also acts as a testing platform for testing websites and an ssh gateway to my network. Other than that you've pretty much outlined the uses, using &lt;5W is a big advantage over anything if you're running a computer 24/7. A lot of people are doing home monitoring these days and it could become the next cult computer for that after the viglen mplc or the linksys slug