Before you can do so, you need to install a plugin from Wacom. As soon as this is done, Colorillo will automatically recognize your tablet and you can simply vary the pen size by pressing harder while drawing. There is also a new FAQ entry about this.
This allows to draw lines with variable width throughout the line, like in the picture in this post. If your pen has a second pressure sensitive point at the other end of the pen, this eraser works too (it erases by drawing in white).
One caveat: the plugin by Wacom doesn’t seem to work in Firefox 3.6 or Chrome, so you may need to resort to an earlier version of Firefox or Safari. Let’s hope that Wacom fixes this soon.
I am no expert with pressure-sensitive drawing on tablets, so please tell me if it all behaves as expected. Happy drawing!