Why they decided to make the menu bar in Leopard transparent is anyone's guess; I have yet to meet one person who likes this "feature". Hopefully future versions of Leopard will include the ability to customize the opacity of this bar so that you can put things back to it being solid. In the mean time though, there are a couple of ways you can go about getting a solid menu bar back in Leopard.

1 - Change your background to a solid color. IF you change your background to a solid color, the menu bar will become solid again. No idea why, it just does. This is probably the easiest fix to this problem.

2 - Making use of the fact that a solid color under the menu bar results in a solid menu bar, you can edit your background image so that the top 23 pixels are a solid color. When you do this, you'll end up with a solid menu bar and you'll still be able to have your background image.

3 - Use OpaqueMenuBar from http://www.eternalstorms.at/utilities/opaquemenubar/index.html. This little piece of software automatically updates your background images to have a solid stripe at the top so that the menu bar is shown as solid. It doesn't alter the original image, it creates a temporary version of the image and makes use of it.

While all of these techniques will fix the transparency in the menu bar itself, none of them fix the problem that occurs when you pull down items in the menu bar. Those will still be opaque and hard to read and that will continue until Apple wakes up and allows this to be adjusted.