It appears Apple is using the same "blade" SSDs with the 2012 iMac that they use with the MacBook Air and MacBook Pro with Retina Display. The MBA and rMBP do use different form factors, however.
OWC does offer their own SSDs in both form factors. As the 27" iMac can have a 768GB SSD from Apple, I am assuming they use the rMBP form factor and OWC's 480GB drive would be $580.