One of my favorite "diff" tooks is "tkdiff" which is a graphical diff using Tcl/Tk. You can find it at http://www.accurev.com/free/tkdiff/index.html When you download just the "tkdiff" script, you'll notice that it is distributed under the GNU license, so it's OK to include.
I don't think we want to add any tk programs to the distro.