MacDevCenter.com: Xcode for the Rest of Us, Part II
This article talks about a graphical diff tool called opendiff, that comes with Xcode. My preferred text editor is TextWrangler, which comes with its own tool. Within the editor you can either click on Search->Find Differences or even better compare the two files on the command line using the command line tool twdiff. You have to have the TextWrangler command line tools installed though. To install TextWrangler->Preferences->Tools->Install Command Line Tools.