Legit question. Were you messing with some wires or did this "just happen"?
Since the TV only has 1 HDMI input we have a 3 port HDMI hub which has the xbox and the tivo (btw, the downstairs TiVo is dedicated to recording all hockey on TV so we won't run into the new year's situation again -- 200 hours of HD hockey). Anyway, we were watching TiVo and went to switch to Xbox and got nothing.
Now, I have tried swapping around HDMI cables, got rid of the hub and tried connecting directly. I also tried component cables instead of HDMI for the TiVo. So, we've tested different video sources (xbox, tivo) and different cables directly to the TV.