It appears that 4850 are infamous for fans going dead on them. When my card's fan stopped spinning, I discovered that although the card worked in all PCIe slots, the fan did not work when the card was plugged into one PCIe slot but worked when the card was plugged into another.
So in at least some of them, this was a motherboard PCIe slot issue?
Intel i7 920  BCLK x 19 = 3.80 GHz @ [1.4000] CPU Voltage & [1.35000] QPI/DRAM Uncore Voltage, Batch 3836A394
3 x 1GB G.SKIL DDR3 1333 (PC3 10666) [DDR3-1691MHz] 10-10-10-24 @ 1.64 DRAM Bus Voltage
ASUS P6T Deluxe v.1 [LGA 1366 Intel X58] BIOS 1606
Thermalright Ultra-120 eXtreme 1366 RT with 120mm Scythe S-Flex F fan
ASUS EAH4850 TOP Radeon HD 4850 512MB @ 680 MHz GPU & 2100 MHz Memory
OCZ Agility 60GB SSD
Antec nine hundred case, two front 120mm fans, one back 120mm Fan, one top 200mm fan
Corsair CMPSU-750TX 750W