before posting this question I just want you to know that I did a lot of googling and came up empty.
I'm on OS: Windows 7, GPU: ADM Radeon 5850 (using AMD Control Center, latest drivers installed). I have one LCD screen that I use for casual work on my PC and I also have Panasonic Viera, that I'd like to mirror my full screen gameplay to (for presentation purposes). Both are FullHD-capable displays (LCD native res: 1920x1080 so is Viera) so there shouldn't be any issue with resolution incompatibility. Viera HDTV is connected directly (no VCR receiver in the middle) via HDMI <-> HDMI.
The thing is that I managed to mirror the Windows 7 desktop with no problem. It shows up nicely, I can see task bar, all the icons etc. (i.e. What's happening on LCD gets duplicated on Viera.)
However, whenever I start a game in full screen mode the secondary display (Viera) goes black. When I start the game in windowed mode it shows on Viera just fine!
What the hell is happening here? Problem persists for all kinds of different games (Diablo 3 beta, Battlefield 3, Mass Effect 3, Gothic 2). I've also tried full screen video in VLC player with no problem whatsoever.