I had no problems installing an running Gimp under Mac OS X 10.4. It's my primary graphics program on my iMac.
On my Macbook with Leopard (10.5), I had to install a version of X11 which was not packed by Apple but by the Open Source community (I don't have the web address at hand but googling for Mac OS, X11 and Leopard should help) and upgrade to the latest Gimp version.
It is still not perfect, especially under Spaces: the Gimp tooltips appear no matter which space is selected. I have to "hide" X11 or quit Gimp to get rid of the problem.