There was a program (Shapeshifter, unsanity.com) that theme'd OS X but it isn't supported in Leopard for some reason.
Shapeshifter doesnt work in Leopard and a new theme change is being developed. i think its been released. check out the MacThemes forums.
Last night, the company announced Architect, a theme building application; and Façade, their theme switcher both for Mac OS X Leopard.
Here's the announcement on MacThemes
Beta is already available!! yay!