Porting X11 Apps to Mac OS X Aqua?

masonbrown writes "I'm fairly new to programming, and have spent most of my time with Linux, Solaris, and IRIX, with most of my learning done on open-source software. As a long time Mac user playing around with Mac OS X, I'm interested in the portability of X11 apps to the new Mac Aqua interface. Is there any straight forward way of doing this, or does the GUI for each program have to be completely rewritten for the new Mac OS?"