QXi (Qt XInput Extension) is a C library to add support for XInput devices (like tablets or joysticks) to Qt-based X11 applications. It's licensed under Troll Tech's QPL, just like the free edition of Qt.
As of now, QXi supports devices with up to six valuator axes in absolute mode, up to 32 buttons, and a virtually unlimited number of keys. Yes, that means that Wacom tablets more or less work already. Use of these devices' capabilities from within Qt is just slightly more difficult than handling the mouse.
Support for relative mode, devices with ridiculous numbers of axes, motion history, and even grabbing is still forthcoming. So is documentation, unfortunately, but then again, if you're not willing to study some source to use this library, then it's not yet for you. That should however change soon.