iTunes for Mac has already been largely rewritten when Apple added 64-bit support to v10.4. It's the interface that's still largely the same old stuff and needs to be updated. Back when the 64-bit version was released almost everyone here on the forum, including myself, reported massive improvements in performance.
I hope it's a complete rewrite and not just a GUI update.
The interface is pretty much the only thing left that truly needs to be redone, on OS X that is.