A simpler fix for those having troubles:
Beside the font-cache clearing technique (which didn't work for me, ever), I must point out a simpler, older method which DID work for me and used to be my common fix for screen-fonts getting jumbled up in various older versions of Mac OS X.
It seems Mac OS X is getting confused over which version of Helvetica to use for rendering menus, titles etc. which leads to jumbled and incoherent text on screen. It may also be a problem with a Postscript version of Helvetica. BTW: Lucida Grande isn't the only font OS X uses for text on menus etc.
If you're using a font-manager like FontExplorer or Suitcase, simply go to your main font library and turn off all versions of Helvetica that aren't system fonts. In other words, any extra versions of Helvetica (including Neue, Rounded etc) that you have installed may cause this problem.
You can tell which the correct System versions are by the lock on their suitcase icon.
This is of course an irritating change if you need to use those other flavours of Helvetica for design work, but there you have it. I'm not sure if this is a problem with the way the font manager handles the fonts or a system error or a corrupt font-suitcase or a Postscript vs True Type thing - you'd need to test for yourself one-by-one.