The X.org Wiki has [a page](http://www.x.org/wiki/XDarwin) for updates for XDarwin. I was still having some issues (with `CURSOR`) that these updates fixed. The two lines that will do the updates are:
curl -LO \
sudo sh x11_update.sh
UPDATE: it appears you should get your updates [here](http://trac.macosforge.org/projects/xquartz) now. I still have X problems, but things are definitely improving.