Do you just want to use the screen as the PS3 display?
or are you looking to control the PS3 from the iMac's keyboard/mouse?
For the former there are USB TV tuners you can buy with widely varying degrees of quality, some of which have a serious display delay which makes gaming through them impossible.
For the latter, just plug the USB cable of the keyboard in or pair the bluetooth
I suspect he means the former. OP, look into the Elgato Eyetv hybrid, as it's marketed for console use. Note that there's a bit of lag so it might not be suitable for online play.
Ah yes, I was forgetting that the 2011 models don't have their Mini DisplayPorts anymore. Although both Kanex and Atlona appear to mention on their websites that they are working on an updated version for Thunderbolt.