Recently an update to Portal 2 included these updates:

Big Picture Support
- Added 2 controller splitscreen support for Standard Co-Op. Press X on the second controller inside the first Co-op menu to activate it.
- Fixed controller's ‘quick ping’ button causing the player’s movement to stop.
- Fixed not being able to exit Robot Enrichment or Create Test Chambers menus with the controller.

This is awesome news for people wanting to run Steam in the living room as adding the same console features to the PC version helps this. Recently Big Picture mode came to my Steam and my first thoughts of it were good. Over the next few days I plan on doing a bender of Lets Plays, showcasing the new Steam Big Picture mode is on my list!