BTW, it's trivial to demonstrate the difference.

On OS X:

$ cd $HOME
$ mkdir test
$ touch test/test
$ chown :musicg test
$ touch test/test2
$ ls -l test
total 0
-rw-r-----  1 nad  staff   0 Nov 29 14:01 test
-rw-r-----  1 nad  musicg  0 Nov 29 14:01 test2

On my linux system, the same test results in:

$ ls -l test
total 0
-rw-r----- 1 nad nad 0 2009-11-29 14:02 test
-rw-r----- 1 nad nad 0 2009-11-29 14:02 test2