The Apple link doesn't show crap about supporting an external display, 30" or otherwise. (at least, it doesn't say anything other than "connect an external display, yadda yadda yadda."
Technically yes, the card can support it, but the iMac hardware can't. The 30" panel requires a dual link DVI, the Mini-DVI connector on the iMac is single link, giving the max digital resolution of 1920x1200.
You'll get a single link of resolution which ends up being 1280 x 800 on the 30" panel.
No to mention this page shows the compatibility of Apple's displays with Macs. Who knew that Apple.com would be the source for the specifications on Apple products?