Ted Patrick has posted a TON of slides from a component developer summit held at Adobe a couple of weeks ago. One (of the many) that caught my eye was this presentation by Gary Grossman (one of main engineers who worked on the new AVM in the player):
This goes into some depth on how the AVM works, and some performance tricks (i.e. casting array indexes to INT).
Make sure to check out the other slides, including presentations on Flex internals, components, and architecture. You can view them all here.
You can view the slides for the Apollo presentation that I did, here.