i have this issue as well.. i found a workaround.. i have the htpc plugged into a pioneer sc05 receiver and it does this all the time.. so i disabled all the sleep functions and all the hybrid sleep functions and all the hibernate functions and left htpc on and just turned off the tv and receiver.. kind of lame but it is one of many problems... i have windows 7 as well so it may also have something to do with that os...
That is lame, but seems to be normal, always have to leave things on because sleep only works the first day you buy a PC!
I also have Windows 7 but I think the problem is in the BIOS or video BIOS
I fixed the problem by buying 6 monitors until I found one that works, maybe by fluke, who knows with HDMI. I have bought so much equipment from HDMI issues I'm beginning to wonder if that's the real reason it was shoved down our throats??