There are 8800 GTX MXM cards available. The one in the picture comes with a laptop but I'm sure it's possible to get one from somewhere.
Now, all we need is to figure out if it's pin compatible (anyone prepared to spend a big pile of cash and risk both their imac and graphics card?), get some drivers (I guess we'll have 8800 series drivers in the next week or so anyhow, but will they work with a mobile part?), and a hacked bios. Oh, and somebody skilled enough to modify the imac cooling system to suit.