I have a Hitachi 55VF820 that I bought in Aug 2006, and my HDMI Input 1 just stopped working as well, like 2 days ago.
Same issue as yours, saying No Source Signal. Haven't tried it on S-Video yet to see if it's that entire input, but I think it's just the HDMI input, as my cable was very loose and kind of just slid out.
On HDMI Input 2, which is hooked up to my DirecTV HD DVR it works fine, but I swap that into the HDMI 1 and nothing so yeah, I think it's just that input.
No biggie, as I'll just use components for my DVD on Input 3 or something. I have a warranty through Circuit City until next August, but not sure if it's worth having the set gone for 3-6 months (probably) just for that one little input, as I mainly just watch movies on HD on my premium package nowadays.
Hitachi, Polk and Yammy user!