I have created a pygtk program to monitor the keyboard status and mouse clicks across any application (i.e. not just for my application). This uses evdev by Micah Dowty which basically monitors /dev/input/event* for output and decodes the information in a way that I can display the information to the user.
The program works perfectly for USB keyboards and mice, but doesn't work for a USB Wacom (nor serial Wacom), even when running as root. I don't get any events for coming out of any of the /dev/input/* nodes for the Wacom as use it. It's not just my Wacom, but also they guys from the podcast "Meet the Gimp" (Rolf and Philippe) don't get anything as well.
The code lives at http://code.google.com/p/key-mon.
To test, you can run something like:
sudo ./evdev.py /dev/input/event0
You should see events coming out for whatever is connected to event0 as you move the mouse or click the keyboard, for example.