1. for multitouch, you can pull in other libraries and override the surfaceTouchEvent() handler as described in the wiki. this stuff by Luke Hutch looks promising:
http://lukehutch.wordpress.com/2010/01/06/my-multi-touch-code-ported-to-eclair/

2. check wiki.processing.org for info about the buttons. I added some additional info yesterday or the day before.

3. mic I haven't heard from anyone yet, but Jer Thorp was getting decent images out of the camera, and I think he's hoping to post a tutorial when he gets a chance.

for the most part, you can just call other libraries the way you would from any Android application, at least until I have a chance to support them in a simpler way using our APIs.