Comment 5 for bug 501266

I just tried to use a UC-Logic tablet and inkscape recognized it properly. However before it registred any strokes I had to go to file/input preferences and select the tablet from the drop-down list even though it was already selected. After that pressure sensivity works as expected.
So maybe tablet driver reinstallation would help? If not, you could just grab a USB pendrive and put the portable version of gimp on it to make sure that it is a problem with inkscape:
http://portableapps.com/apps/graphics_pictures/gimp_portable

Also note that the calligraphy tool has a pressure sensitivity toggle button (next to stroke width slider) that is separate of the preferences setting.

Tested on Inkscape 0.47 r22583