Open up a PDF document in Preview. Now, move your mouse over the body of the document.
Press the accent/tilde key, and a magnifying glass will appear. You can drag it over the document to zoom in, and then dismiss is by pressing the accent/tilde key again.
[crarko adds: There's also a corresponding new 'Show Magnifier' menu item in the 'Tools' menu of Preview.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110921055816944