Back when I worked at LimeWire (probably the largest Swing install base in the world) I played around with MacWidgets. It's really an admirable effort, but trying to emulate Cocoa in Java always feels a little off. It's sort of an Uncanny Valley effect...you can't easily point out what's wrong, but you know it's not quite right.

Either build a real Cocoa interface, or go LimeWire's route and don't make it look native at all.