I just got my copy of Leopard, and while I must admit that I am half-happy with it, there are a few interesting new features.
Among them a small application named iSync Plug-in Maker, included in the Developer tools (/Developer » Applications » Utilities). The name says it all! It probably does little more than what we did in Tiger by hacking existing plug-ins, but at least it's a clean way.
[robg adds: I took a look at this app, and it's quite polished; it even includes image wells for dropping in photos of the new device to use as icons.]