Transparency is still used quite often in OS X. For example, all menus are semi-transparent, as is the Dock.
In earlier version of OS X, the title bars of inactive windows were also semi-transparent. This was done away with in 10.3, IIRC. It was never a very useful or attractive feature, so I'm glad they ditched it.
Vista seems to be taking this a step further by making even the main body of even active windows semi-transparent. Go figure.
I don't think you can really say MS is copying Apple here, though, because Vista's effects are really a step above anything Apple's done, or even could have done when OS X was first released. Vista's visual effect is actually pretty neat, with the light streaks, slight blurring and tinting.....but as with OS X's old semi-transparent title bars, I don't think it's very useful.
That said, I wouldn't be surprised to see Apple use more dynamic visual effects in Leopard. Before Tiger it would have been sickeningly impractical, but with Tiger's Core Image, the performance hit wouldn't be much greater than the transparency and stuff we see now.
