i just got the 2011 imac... and they have (7pin) proprietary connections now (sata). = so, you are unable to upgrade the hdd on your own.
apple basically forced me into buying a 2tb+256gb (ssd).
i plan on investing in a thunderbolt raid array but i count on that being a pretty penny so for now i have this new machine with a significant amount of money already invested in it.
another problem: apple doesn't support ssd caching. with the new intel z68 chip i was hoping apple would take advantage of this technology to increase a significant boost in os drive performance.
but to get back on topic... you can only have dual drives sata+ssd with the 2010 and forward imacs. and even then, apple is discouraging upgrades outside of the initial purchase.
another thing to add about performance: i installed logic pro on this new machine and loaded the "Spaceman" (the killers) session and i had the buffer set to 32 and played the entire song about 6 times. only 9% cpu usage!
it was amazing considering the session is roughly 80 tracks including auxes/automation/inserts/buses/video content etc. apple really outdid themselves on this new imac!