use MBP as external screen
I've researched this topic myself, and everything I've found says that the HDMI port and/or Thunderbolt port on Macs is not meant as a display input. You can't use it for that purpose.

However, something that may work, if you're willing to spend money, is a capture card. I've used them for recording video game footage before. The one I use has a noticeable lag. It's split from the TV to the capture card, and what happens on the TV takes 1 or 2 seconds to display on the input from the capture card.

If you could find one without lag, or with very minimal lag, it'd be usable for gaming purposes. Of course, if you're trying to hook up something that doesn't need to be lag free, such as a movie player, or anything else that plays non-actively controlled media (actively-controlled media would be video games, and pretty much nothing else), you could do with a capture card that has some lag.

This is a capture card that advertises OS X compatibility: http://www.blackmagicdesign.com/products/intensity/

It has probably any input you could need. I'm not sure if it lags or not, as I have not used it. Also, you have to consider the fact that they may use proprietary software for recording. If they do, and there's no way to display a video stream without recording, it's probably not a good solution for you. 

Some quick searching seems to indicate there is no noticeable lag with Blackmagic's Intensity capture cards. It also seems to suggest that their software can display without recording. I don't know if you can full screen it or not. Or if you can display a video stream in a program of your choice.

Edit: I just noticed on their site, they said it supports streaming. And doing so with "all leading streaming software" too. Which would suggest that it does provide a video stream, which you could utilize in a program of your choice, and also display in full screen.