I didn't see this documented anywhere, but in the latest version of Apple's X11.app (v 0.2), one can copy text from OS X applications and paste it into XWindow applications with Control-V -- rather than the usual Command-V keystroke. Cool!
The documentation suggests that in the previous version, the middle mouse button would initiate a paste, but I could never get it to work properly.
[Editor's note: If you're copying out of X11, the usual Command-C works (as Control-C has a different meaning in most UNIX apps!).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030211183902429