Well, this is an old thread, but the REAL reason they get hot is because the manufacturers that MADE them did not account for using them in SLI mode.
I put a fan behind my 570's, no change in temperature. I tried an 80 and 120 mm fan blowing directly on the cards in front of where their fans are (middle) and it only dropped the temp a few degrees.
However, I got a fan out of an HP (or IBM, I forget) server, it is only about 60mm and damn loud, but playing BF3 my temps are really close around 75 for the top one and 70 for the bottom one. Idle is around 45 and 32 (looking at evga precision right now)
I am going to go water cooling only on my GPU's, not CPU's (yes, I have the evga SR-2 with 2 6 core Xeon's) Or, I suppose with this board I can space the 2 cards out some. Right now I am running my setup on the bench till I get everything figured out.
I still blame the manufacturer, and NVIDA for the reference design. The best cooling would be to force the air from the rear with a squirrel cage fan of some sort, if you have room.
While 104 may be the design spec maximum, I am a little worried about long term life at high temperatures.