AT&T UVerse boxes run Windows CE. I was surprised to see this when I browsed to the details of the OS on my box.
It runs surprisingly really well. It's an IP Based tuning system (IPTV). Currently it supports 4 concurrent streams i.e. you can record 4 streams at once (mix / match watch vs. record). This is purely limited by bandwidth. With the previous UVerse profile (before they did a firmware update), it supported 3 concurrent streams. Once AT&T bumped their profile, the box automatically supported 4 profiles.
I've had no problems recording multiple HD streams.
The box also supports playing DLNA / UPnP music from a media server. I had hoped that at some point they would also support DLNA video streaming, but I suspect they don't due to it cannibalizing their VOD / pay features.
Combine the features they support on 360 with what they do via the UVerse boxes, they could come out with a kick ass media box.
Do you guys know what you're doing, or are you just hacking?