Softimage now disables tablet pressure support to prevent problems with tablets.

There is strange user interface lag that affects Softimage XSI when you use a Wacom tablet. With heavy scenes or simulation, you may never be able to stop playback. Other users have reported problems with the render region.The best work-around is to disable tablet support in Softimage any version. This will not affect any other program.

via Softimage Blog » Blog Archive » Work-Around to Problems with Tablets and Softimage.