The famed annoyance of the translucent Leopard menubar has finally been solved. Steve Miner has posted a tip that involves changing an environment variable by editing a plist to make Leopard think it's running on an older Mac that doesn't support the translucency. Once done, it will make your menubar solid white. The guys at Many Tricks (of Butler fame) take this to the next level with Menu Bar Tint, which places a pleasing tint gradient over your now blindingly white menubar, and thus returning your Leopard desktop to harmony. So there you go, if you just what an opaque menubar, run Miner's trick; if then want it to look better, have a look at Menu Bar Tint.
Update: Gruber points out that commenters on Mac OSX Hints have discovered that Miner's original tip includes a kind of scaling factor. Apparently a setting of 0.63 gives you a greyish menubar "like the opaque menu bar Leopard shows on systems with older video cards." Doing it this way does not require you to have Menu Bar Tint running all the time, and yet apparently still gives a gradient (see below).