That setup you described above is perfect, just perfect. Truly worth building. I still however think it needs a high-end remote, perhaps a chinese clone phone with the camera flash module modded to be an IR LED and moved to the normal position. But that's just me.
In any case, great work.
Upgradable, off-the-shelf components are a definite yes, and something i've believed in from the very beginning. I've already been thinking of either a compact, "Mac Mini"-style motherboard, or something flat and thin, like a rack server... nothing in particular, but running Luna on top of Ubuntu really does solve the issue of drivers and compatibility
I really do believe we can build something here that the big manufacturers can't...
Edit: another (unrelated) thing i've thought of, is that a media center PC can act as a print server for mobile devices, and a WiFi router board could be used (optionally) as a power-saving measure (to wake up the main board when needed) and independently handle file storage and backup... the router component could even act as a home base Synergy server for phones and tablets. I realize that this is an added expense and a little "out there", but that's extra stability and extra capability.
Also for reliability, an emergency recovery image could be booted automatically if a problem is detected and the current OS installation needs to be repaired. These are all high-end features that most manufacturers won't be willing to provide for another decade