It's no secret that the transparent menu bar is unpopular. Since I keep a bare minimum of stuff in my menu bar, I decided to get rid of it altogether. The Info.plist trick detailed in this hint works in Leopard. Note that somewhere in the comments it's pointed out that you need to move the app before opening it again with a hidden menu bar to bypass some cached files.
Much to my amazement, though, when you mouse over the menu bar after using this trick, the menu bar reappears fully opaque. Obviously this isn't a solution for everyone, especially menu bar junkies, but hopefully someone that knows more than me can figure out why this happens and how to make it the default.
[robg adds: I tried this on the 10.5 Finder, and it definitely creates an opaque menu bar. As with 10.4, though, Spotlight breaks. Still, perhaps some dtrace expert can figure out why the menu bar is coming back opaque when used in this manner. Sadly, the menus themselves are still translucent.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071102080028265