As explained here, the very first Leopard hack, namely Non-Transparent Menu Bar, was forcefully retired somewhat prematurely. So we recycled its source code and turned it into another nostalgia hack.
See, the new Mac OS X doesn’t draw rounded screen corners anymore. Most users won’t notice, others will be happy about this change, but some might miss that cuddly CRT aura those rounded corners emitted.
So if you’d rather still have rounded corners, have a look at Displaperture, the little tool that lets you retain your beloved rounded screen corners. You can even determine those corners’ radii, and you can pick the corners you want rounded — including the bottom ones.