Plug a Mini DisplayPort cable into both machines, hit Cmd+F2 (as far as I recall) on the MBP.

This is already built-in: http://support.apple.com/kb/PH14264

Woah, do you know if this works with the new retina display iMacs?

No, sorry to disappoint you!

http://support.apple.com/en-us/HT3924

At the risk of answering without a guaranteed answer, "probably not," given the amount of the power necessary to drive that screen and complex architecture inside the box to make it work. But Target Display Mode as a thing has been around for a while.

I'm here to confirm to you that it doesn't. Sorry buddy, wish it did myself. 5K is out of reach for most MBP's (if not all).

That's a bummer, it would be worth buying a new iMac for the screen alone.