Since the iMac uses a laptop graphics GPU, it's not on a card as you know it (I'ts a really small card - daughtercard size), it's more of a module. So no there is no fan on either of the graphic chip modules. There is no space for such a thing. The iMac cooling system is probably responsible for keeping the graphics GPU cool. Maybe there's a heatpipe or heatsink specifically for the module.
Here is a link of what it would look similar to: http://www.hothardware.com/articleim...ll_module1.jpg
-- Mac Pro Dual 3.0 Dual Core, 4GB RAM, 150GB Raptor HD, 500GB HD, Superdrive, Radeon X1900XT, Bluetooth, Airport Exteme, Logitech Z-5500 5.1 Speakers, 30" ACD
-- PowerMac G5 Dual 2.5, 2.5GB RAM, 160GB HD, Superdrive, Radeon X800XT, Bluetooth, Airport Extreme
-- Macbook Pro 2.53 15", 4GB, 320GB HD, SuperDrive, Bluetooth, Airport Extreme