The wifi one charges fine on my Mac Pro, though it has to be connected directly into it, not into the USB ports on my monitor. Not sure why it wouldn't charge on your iMac.
Plug it directly into a USB port Jon the iMac. My WiFi, & 3G iPads charge fine from my iMac. And it doesn't matter what sync cable I use; iPhone, iPad, or third party it charges fine.
My 24" iMac from late-2006 doesn't charge my iPad; however, my MBP from mid-2009 charges it fine. As stated previously, it's an issue with low-power USB ports.