Is Portal 2 Talent Show Achievement disabled?
Are you using the Sixense MotionPack? If so, that might be your problem.
Fix:
It is possible to disable the motionpack by adding the launch option "-nosixense" in Steam. (right click on Portal 2 in the games list, select Properties, then Set Launch Options...)
EDIT: Valve released a patch on October 11, 2011 which solves this issue. Change log of that patch:
- Fixed the Talent Show achievement not being awarded properly