It's been hard to find a good set of AS3 coding standards in the past but this fits the bill. It's pretty thorough even though there are a few sections still marked TBD. It covers naming standards, language usage, file organization, and code formatting.
The standards all seem pretty reasonable too, unlike some other standards documents I've seen elsewhere. I'd say it's about a 95% match with my personal AS3 coding preferences.
Even if you don't agree with all the standards, this is a great starting point for any project team that's embarking on a big Flex, Flash, or ActionScript project. You can just use the Adobe standards as a baseline and tweak as necessary.