Here is what I had to do to enable tablet pressure support for the X11 based Gimp and Inkscape applications on OS X Leopard:

My setup:

• OS X Leopard: 10.5.5 on Intel
• X11: XQuartz 2.3.2 RC2
• Inkscape: 0.46 (Mac binary)
• Gimp: 2.6.3 (Mac binary)
• Tablet: Wacom Graphire ET from around 2003

Key points are:

• get at least the 2.3.2 version of XQuartz. 2.3.1 will not work!
• In Gimp/Edit/Preferences/Input Devices: Click on “Configure extended input devices…”, set device ‘pen’ to ‘screen’.
• In Inkscape/File/Input Devices …: Set device ‘pen’ to ‘screen’.
• You might have to restart your apps

Nice. Now we have pressure support on OS X for these two great apps. The Inkscape calligraphy tool is very nice. Makes my handwriting look nicer than on paper.