Here's another excellent link on the subject. The article also has other links. Gail Shaw is a Microsoft Certified Master and she has put together quite the writeup on these subjects including some of the caveates of using one vs another.
http://sqlinthewild.co.za/index.php/2010/02/18/not-exists-vs-not-in/
--Jeff Moden
Change is inevitable... Change for the better is not.