Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_4 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8K2 Safari/6533.18.5)
It's hard to say because I haven't owned an iMac, in fact I just placed an order for one. On my MacBook Pro I have the same issue as you. The fan noise bleeds into the microphone causing terrible audio quality. Perhaps you could lower your fan speed with a fan controller during skype calls.