I found
this little trick by performing a simple Google search. From what I gather, you'll need a copy of Tiger's Dock.app file, and do some poking around in the Terminal. Also, the author claims that the running indicators do not show up properly, or at all, so if that's a problem for you, you may wish to reconsider. There are obviously risks involved, so be sure you can take the loss if the worst case scenario should arise.

I don't see why you'd like to replace the dock in Leopard. I'd honestly trade my Tiger dock for it if I could, but to each his own.