I had a fan on one of my 5830's that was clogged up with a cable, without me realizing it.  It stayed under 95c by clocking down to 300/150 until it cooled a bit, then clocking back up to 600/300, then back down, all automatically.  I was wondering why my hash rate was so low with that particular card.    But basically, the card will still survive just fine even with a non-functioning fan.  I'd say the only risk is if the heatsink itself becomes detached from the card, but if that happens without you realizing it, there's probably other issues with your computer that you need to work on anyway.