You can do exactly what you want with MCE 2005.
Buy or build your main HTPC with MCE2005 then go buy an extender for it. There are a few out there, I know Linksys has one and the xbox and xbox360 can act as an extender. Basically what they will do is act as a front end so if you have stuff recorded on the main MCE box the extenders can play it back.
I currently have x2 SDTV and x1 HDTV tuners in my MCE box. I've been using all 3 tuners to record and played back a show I had recorded with no trouble at all (6600 & P4 3Ghz HT). One of the Microsoft MCE developers has a well know blog where he said he had something like x8 tuners running in his box. Just a easy registery setting change to allow it (and the hardware).
HTPC is deinitely a hobby to most but its quickly becomming a bit more mainstream. Microsoft is really driving it right now, I can't wait for Vista MCE.
