By default, double-clicking a folder in the Mac OS X Finder opens the folder in the current window. You probably know that you can open a new window for a folder that appears in the current window by holding down the Command key before double-clicking. You might also know you can open the parent of the current folder in a new window with Command-Up Arrow -- but this will close the original folder, too.
To open the parent folder of the current window in a new window, while leaving the current window open, hold Command and Control, and then press the Up Arrow key on the keyboard.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081003092141751