Thanks for the response tacit (and sorry for the slow follow-up on my part).
To clarify things a bit:
1) This ugliness happens with jconsole on both my macs (one running 10.6, the other 10.5.8 Server).
2) Other X11 apps (such as emacs) display just fine on both macs.
So, based on your response, it seems that it's likely an issue where jconsole is using a font that's not one of the standard fonts installed with the Mac OS X X11 server. What I can't figure out is which font it is, and, once I do, where to get a copy of that font. I've looked for log files, and in console.app, with no luck.
Any thoughts would be appreciated!