Portal/Mortal Kombat mashup features fatalities that utilize the Portal gun. (these are pretty violent.)
P-Body makes an appearance in the single player campaign.
On test chamber 16 I found if you destroy the turret in the vent, it will explode and you can enter. Through the grates at the bottom you'll find 4 sentry guns who will play a little song for you =).
How to get the achievement "Break the Rules" in Portal 2. (A sad little moment with a companion cube.)
Via Kotaku, the extras menu features an interactive trailer for Super 8 - - you walk through the trailer as if it was a Portal level.