Not sure if too late, but I went through 2 GTX 460s (EVGA) with the same issue. I never did find out what was the cause.
The cards played games fine (would however TDR during gaming sometimes) but after reformatting, try the handful of drivers available and 2 RMAs EVGA gave us a GTX 460 SC for the difference (<$20) and it stopped.
Until I updated her drivers again (forgot which version) and the TDR issue came back. When I swapped her to the temp GTX 560 Ti the problem persisted, it would always involve her ALT-Tabbing to Firefox, that much was a given.
Someone somewhere said it was a power issue from the cards switching clocks due to various different states. I think it would be related. For example, now with my HD 7970, if I load up Youtube my card locks to 501mhz which creates gaming lag, if I close the Youtube page it clocks back up to 1150mhz.
I think these cards are getting too sophisticated and unfortunately, we'll run into errors along the way. Thankfully, the GTX 680 has yet to TDR on us (knock on wood.)