I was just trying Puzzle Agent through Steam on my new iMac, which has an nVidia 660M card, and I had the (now I know) famous bug with the Puzzle nº2. I searched in the forums and I saw that an update was released for the 4XX series, but I just downloaded it today from Steam, so it should be updated already...
Seems like the bug has appeared again with the 6XX series? Honestly, I have no hope of it being solved, at least very soon, seeing that it was so hard to do it previously. But I'd like to know that people from Telltale are aware of this issue!