I just set up a new workstation with a 5870 card and 2.1 SDK. Before I installed the SDK 2.1, the card ran normally but after I installed the SDK, the card's fan was boost to > 2500 RPM which is 2 times in normal mode and it sounds very loudly. Note that the card fan runs at that speed even I don't do anything (activities ~0%) so I think the SDK 2.1's driver cause some problem on fan speed control and/or power management of the card.
Luckily I discover one solution to revert the problem: go to Catalyst control center->ATI Overdrive then set all the setting of the card back to factory default (press Defaults) then the card's fan back to normal mode.
I'm not sure this happened to anyone before but just a reminder for you guys to double check your card's fan speed because sometime we just simply don't care about how is our cards running inside the PC chasis. And if it is a wide-spread problem, ATI should check and fix it.