I always forget this, thought it’s trivial.
To change the program you use for diff-ing, for editing of the commit logs, and much more, edit the ~/.subversion/config file, e.g.
editor-cmd = vim diff-cmd = meld
The file is well commented, so you should find your way around it easily.
Note: with update to Ubuntu 11.04, meld stopped to work as an SVN diff program for me (seems like support for ‘-v’ parameter has been dropped); this can be solved easily – just create a new script, e.g. /usr/bin/svnmeld containing:
#!/bin/bash meld "$6" "$7"
then chmod it as executable, and use diff-cmd = svnmeld in your SVN config file.