Macromedia's Flash editor has a pen tool where you
select a pen "tip" (ellipse or rectangle). As you draw
a path with the tip, the editor displays a realtime
bitmap preview of the pen's "ink" trail. The ink
trail's radius increases with increasing stylus
pressure (on my Wacom tablet). When you pause, it
calculates/interpolates the edges of the tip along the
path and replaces the bitmap preview with a smooth,
blobby vector object.
I love this drawing tool. It's great for cartoons.
p.s. Sketch is already fantastic. Very solid. Well done!