I deal with a lot of PDF documents and until now was using Lion. I used to Quick Look PDFs all the time, and pinching used to make it full screen. But even in full screen, because most document have a portrait orientation, many times the text wasn't readable enough. Now, pinching in Quick Look, both normally and when full screened, zooms in or out like in Preview. I've tested this only with PDFs. I tried with a JPG, and pinching did nothing to it.
[kirkmc adds: Yes, I get this to work with PDFs, but not with other graphic formats. ]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120728171658256