I upgraded (complete blank hard drive re-install) to Mac OS 10.5.6 today, also installing X11 XQuartz 2.3.2, and GIMP 2.6.6 from the websites.
Creating a new blank canvas I was able to draw with the trackpad as expected, but if I set any of the 'extended devices' - pen, eraser, etc - to anything other than 'Disabled', the pointer stopped drawing anything (although it moved around). It started drawing again if I set everything back to 'Disabled'.
It behaved this way whether I had my Wacom Graphire 3 USB device plugged in or not, or whether I tried to draw with it or just the trackpad. Needless to say, no pressure sensitivity (which I understood was believed working).
I tried installing the latest Wacom device driver, just in case that made a difference - sadly not. I hadn't installed any other Wacom-specific software since reinstalling MacOS from scratch.
Bizarrely, after a couple of stop/starts of X11 and GIMP and with some tweaking of the tablet preferences but leaving them essentially unchanged, the pressure sensitivity started working but the trackpad still won't draw if any of the extended stuff is set - even the pointer or cursor devices.
I'm delighted to have pressure sensitivity, but unnerved that it needed jiggling.