No Output via HDMI from Mac Mini

Hi folks,

I've had a Mac Mini (last gen, with the optical drive) running perfectly as an HTPC for quite some time now. Today when I booted it up, nothing displayed on my screen (a Samsung TV, connected via HDMI).

Oddly, if I plug a monitor into the miniDVI port, the HDMI starts working. I can then unplug the monitor and use the system normally over the HDMI connection. If I reboot, however, nothing comes up.

Additionally, screen sharing works just fine until I reboot. I know the machine is up because I can browse it's shared folders from another Mac, but I can't connect with screen sharing, or get the display to come up. The machine is running Lion, if that matters.

I'm not entirely sure how to troubleshoot this - any insight would really be appreciated. Thanks!