When I tried my new graphic tablet in Gimp under Mac OS X, I noticed that it did not work there. After searching the web for a while I read nearly everywhere either that tablets won't work with Apple's X11 at all or that the work, but not the pressure sensitivity.
Luckily, all this information was outdated. To get a graphic tablet working in Gimp under Mac OS X is really easy. All you have to do is to install the newest XQuartz and Gimp versions (for the links see below).
Once this is done you go to Edit > Preferences > Input Devices > Configure Extended Input Devices in Gimp and set the devices pointer, pen, eraser to the Screen mode (the names and number of your devices may differ, if you have another tablet).
My Wacom tablet is working very well now!