This is a really interesting thought. To clarify, you're talking about buying an HP or Dell Cablecard system, already all CableCard ready, and then pulling the guts out and putting them in a nice media center case? It seems reasonable.
Well, the Dell XPS is a BTX system. I don't know a lot about the differences between ATX and BTX, so can you put a BTX motherboard in an ATX case? Either way, I believe the HP computer would be much more doable.