The new X11.app Version 2.0 introduced with Leopard has a lot of bugs. Some of the issues (and workarounds) are discussed in this excellent thread over on the macosxhints forum site. Also, the people from x.org have released updated X11-libraries and an updated Xquartz server; you can read about them here.
[robg adds: The thread on the macosxhints forums contains a ton of information culled from various sources about what works and what doesn't work, and how you can fix some (but not all) of the issues. ]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071122121537522