I can confirm that the ZOTAC ZT-40404-10P SYNERGY GeForce GTX 460 (Fermi) 768MB (http://www.newegg.com/Product/Product.aspx?Item=N82E16814500173) works just fine with the iMac DisplayPort including outputting sound. I'm very pleased so far. :)
Anything needed to get that second 460 running? Running in SLi? How bad is the noise with the Zotac? - a few of the reviews really mentioned that being an issue.