I was under the impression that apple copied the seemingly transparent interface goodies from the guys over at redmond but this screenshot that i found begs to differ:
Quite interesting that OS 10.2 had transparent finder windows (in the background), got rid of them in 10.3 and brought back some transparent effects in 10.5

Quite interesting that OS 10.2 had transparent finder windows (in the background), got rid of them in 10.3 and brought back some transparent effects in 10.5