I have this problem as well and was pretty sure it had nothing to do with my hardware. Until yesterday. For lack of being able to play Skyrim I became a little nostalgic and installed Serious Sam HD. Played about an hour and then I got the same hard crash I've been blaming Skyrim/Drivers/whatever for.
I quickly found a thread in a german forum where a poster stated that HIS produced faulty Radeon cards. So if you have a
PCIe HIS HD 6870 IceQ X GDDR5 1024MB Native HDMI 2xDVI 2xMiniDP with a serial number lower than H110808601 (like me)
or
PCIe HIS HD 6870 IceQ X Turbo GDDR5 1024MB Native HDMI 2xDVI 2xMiniDP with a serial number lower than H110808701
the cause for the crashes is probably your card. This isn't official and I haven't confirmed this with HIS yet, but my retailer will be sending me a replacement once I send them the supposedly faulty card.
link to thread (german): http://www.computerbase.de/forum/showthread.php?t=944563
Going to quote my lazy self here.
It turns out that my card was the cause of my black screens. I've been playing crash free for over 10 hours now. At first I thought the black screens turned into CTDs, but I've only had two of those.
My retailer (Mindfactory from Germany) sent me a replacement immediately, it was a known issue for them. I don't know if the serial numbers I quoted apply for all countries, but if you have one of the mentioned cards, check the number and maybe get in touch with your retailer (and maybe HIS?).