I use my SheevaPlug, in conjunction with an HDHomeRun
tuner box, as a recorder for digital OTA television. It is perfectly capable of recording two shows while I stream a third (or stream one of the ones actively being recorded -- great for skipping the commercials), so I think it would work fine in this capacity for streaming movies. I have never tried to convert it to a MythTV back-end, but others have, I think, with success. Of course, a front-end machine requires a whole lot more horse-power, and I doubt if the Marvel CPU would be up to that task.