I had a problem today, where my Wacom Intuos3 tablet was not working in AE CS5.5. This is in Windows 7 64-bit. When using the Brush tool, none of the tablet-specific features like Pen Pressure or Tilt were working. The pen acted just like a mouse. In brief, I found that AE seems to be binding to the Wacom Mouse driver before it finds the Wacom Tablet driver. I came up with a workaround:
I wrote up all the details, which you can find here if you like: http://aaronkondziela.com/2011/12/wacom-pen-pressure-problem-in-after-effects-cs5-5/ . I also filed a bug report with Adobe, as it seems that it may be an issue in AE.
Hope this helps someone out! Pressure is essential for the brush tool, makes life so much easier.