I'm now in college and my middle school had a program like this. They gave out the laptops by grade, first the 9th graders, wait 2 weeks then the 8th graders ect... When they first handed out the laptops the only thing that was locked down was installing permissions and the school district filtered the internet. The net admins could "see" the screen of any laptop while at school and lock it remotely. They used the teachers to decide what was productive and what was distracting. If a program was found to be a distraction they blocked it. iChat lasted less then a week. After 4 weeks they had a configuration that they would keep for the 3 years while I was there. I thought this system went very well, besides the fact that a hidden admin account had the default Mac password, so installed what I wanted anyway. I know this is /. so it goes without saying, but consider open source software, my school didn't, so I had a crappy $1/license "Think-Free Office" that they banned us from using since it crashed the computers.

MythTV frontends only need a about 667 MHz P3 to render video and most of them have s-video, they are small and use little power so they are perfect for descrete media comps.