Well I'm gonna try putting a crucial M4 in my 27" 2011 tomorrow and let you guys know how it went. I have a friend who owns an Apple repair shop so he has the screen removal cups.
Let us know how it goes. I have a Samsung 830 256GB SSD drive waiting to be put into my early 2011 iMac 21. I have done the upgrade on my early 2011 iMac 27, but only by keeping the original HDD and adding a SDD to the 2nd SATA port.
Looking over the answers in this thread, i'm amazed at how much compromise iMac owners are willing to accept. External drive??? The beauty of the iMac is it's all in one design. Who wants a "wart" hanging off all the time just to add a drive that should be able to be installed internally? Wunderbolt? Yea, right. By the time we ever see affordable, stable Thunderbolt enclosures, the iMac 2013's will be out with USB 3.0. Like others, I feel Apple did it's customers a big disservice not including any industry standard high speed external ports in the 2011 iMacs (eSATA, USB 3.0), and instead, handing us a magical port with no peripherals. Weren't manufacturers given the specs early on so the development would be done at launch? Here we are, almost 1 year later, and still there are, what, two overpriced devices?
As I have all but switched from Windows PC's, with their plethora of high speed ports, to iMacs, it's frustrating I am having to settle with USB 2.0 and FW800 for my "high speed" external needs.
And in the end, Apple has restricted us from putting our own SSD's in these wonderful machines.