I bought an Ion box to do just that. Got it set up in a few days running off of an SD card. Once I get an ethernet cable run, I'll be moving it to PXE booting. Either way, the only moving part is a case fan, which is a little louder than I was hoping, but is still quieter than the fan in my DLP TV set.

I do have a couple of problems. The interface sometimes gets bogged down, but I'm 99% sure this is due to the SD card being slow during filesystem syncs. And my HDMI audio driver doesn't like mono.

All in all, though, I'm pretty pleased with it. I spent more on it than a Boxee Box would have cost, but I'm left with a system that I can reuse for other things. And that is, you know, regular-shaped. (I wonder if a third party will produce a stand for the Boxee Box that will replace its missing corner.) Plus it was a fun little mini-project.