It doesn't interpolate between speed very effectively. So when I transition between a fast stroke and a slow stroke, there's this strange bump when the stroke changes width.

I'd suggest...

  "Variable width signatures (this is implemented very 
   crudely, and I didn't discuss it in this post. The first 
   signature pad in the post uses it though)"

never mind ,I think you and I are on the same page.

Looks fantastic.