That's exactly what worries me with big monitors @ 1080p resolution - it might look like good old 640x480 on a small monitor...
In my limited experience your fears are unfounded. Windows desktop and WoW Cataclysm are sharp and clear on my 50" plasma at 1080p even as close as 6 inches. I sit about 6 feet away and it's perfect.
HD TVs are designed to be sharp (otherwise what's the point) so as long as you run your PC resolution at the TV's maximum native resolution you're fine.