On my tablet, and I believe all the new ones coming out, there's both a pen and a touchscreen. To avoid accidentally writing something while rest your hand on the screen you usually have disable touch completely in the OS. I wrote up some code that turns it off in Xournal, instead.
The effect is that it doesn't write anything if you touch somewhere on the canvas, but you can still press buttons and the like.
There are still some glitches, though. One is that it will occasionally still write, until you use the pen. That gets fixed by turning on discard_corepointer. The other is trickier. Occasionally you won't be able to select any UI element with the pen, until you touch somewhere with your finger. I haven't figured out what triggers it, yet.
That happens rarely though, so it's actually usable.
I've broken into 3 patches against CVS head---the first one adds the option, the second one adds a menu entry, and the third one adds a menu entry for discard_corepointer (since the two options are best used together).