It would probably look great if you did a front to back sort based on droplet size, and resolved collisions (which you'd have to detect) by deleting the smaller droplet.
The Thomas algorithm (http://en.wikipedia.org/wiki/Tridiagonal_matrix_algorithm) and that technique could probs make your dream come true.
I know, just me being fussy. Impressive use of canvas though, it could be really nice as an appeasing background image on a site. Using the navigator geolocalisation and a weather site we could have a weather sensitive site. Have to test resource wise and see.
Source code at http://github.com/premii/hn Or This is another awesome HN app. Scroll down to bottom for source code. http://hackerwebapp.com/
http://maroslaw.github.io/rainyday.js/
(click on the images to see each demo)
