The Perpetual Testing Initiative now supports community-created co-op test chambers as of today's Portal 2 update.

Restart Steam if your copy of Portal 2 isn't updating yet.

Be on your toes - the DLC is coming out sometime today.

http://www.youtube.com/watch?v=b7rZO2ACP3A

The Perpetual Testing Initiative DLC will be released on May 8th to PC and Mac users.

A huge update coming to Garry's Mod sometime this year will be delivering somewhat functioning Portal 2 content support. No word has been given whether the maps from Portal 2 will work as Left 4 Dead series maps are evidently still unsupported.