I woul;d not use mini-itx as I would want more than 1 or 2 hds. I would want to be buying 500gb HDs every couple weeks till my space totalled outrageous amounts.
Bluetooth (what wiimote uses) is not line of sight...your media center pc could be anywhere. I would put it under my chair. MythTV runs on really low specs, so I'd save money there and just buy HDs. Plus you'll need the bluetooth card, not sure how much those cost.
They make "Media" cases for PCs that look alot like Tivos...but I wouldn't want that...like I said...I'd use a full tower and lay it on its side...
Could fit a lot of HDs into that thing.