This is quite weird. Everything works well with my bamboo tablet (both on XP and Vista) except sometimes I can't draw anymore. For example I select the pencil tool. The pointer changes (an arrow with a little pencil on the bottom right) but sometimes there's no circle under the arrow corresponding to the size of the pencil. I can move my pencil around but I just can't draw. If I then move my normal USB mouse (not the one that comes with the tablet), at once the pointer displays the little circle and I can draw. If I move again my pen it disappears again.
Sometimes it happens at Gimp 2.6.7 startup, sometimes it just stops working after some time (on my XP notebook I have no mouse, so it can't interfere with my pen, but I still get the same problem).
Usually restarting Gimp fixes the problem (at least temporarily) but it's not really fun.
Any idea of how to fix it in a more user friendly and long term way?