I also wanted to share an interesting video that my friend sent me. The video is about a Portal-type game created in the Unity 3D game engine, using a different approach to the portal gun we all know and love. In this game you take a snapshot of the screen, and then project the snapshot onto a wall. This allows you to step through the projected snapshot into the area you took a picture of. It's kind of weird to explain, so here is the video:
What's really interesting is that the portals allow you to travel back in time to the moment you took the snapshot. I hope Valve is keeping an eye on this, it would be a great feature to have in Portal 3!