actually, the darwin kernel does recognise both colons and slashes as path delimiters and converts from the one to the other depending on the environment. The Finder doesn't allow you to use colons in a file name and the terminal doesn't allow you to use slashes, but the inverse is true; the Finder allows you to use slashes and the terminal does allow you to use colons. In fact, slashes in the Finder are shown as colons in the terminal and vice versa. The actual magic is done by the kernel however.

If you want the dirty details, Wilfredo Sanchez, the guy who wrote the kernel bits to do this, has a long paper on it on his site: http://www.wsanchez.net/papers/USENIX_2000/