I'll write this up now since I was on vacation last week.
As you probably know, last week we released the beta of Flash Player 10, code named Astro. While we're waiting for Flex 4 to take full advantage of Player 10, we wanted to make sure that developers had some options to try out the new features. Therefore we have a nightly build of the SDK that can output to run in Player 10.
The following pages should be useful:
Lee Brimelow's video showing what to do
Flex SDK instructions
FP10 ASDoc
Note that the best resource is still the page on Labs since as more demos come online we'll link them there.
Also, Flex Builder code hinting will not work for some of the new properties on existing classes (new classes should be OK). We're going to figure out the right way to solve this and it will obviously work in Flex Builder 4, but we're not sure if we'll address it in a Flex Builder 3 patch.
Thanks for that last bit of info I though there was something wrong w/ how I setup the FP10 .swc, that resulted in bad code completion. Also I made a guitar tuner w/ Dynamic Sound and Flash Player 10. Check it out: http://tinyurl.com/5tjgro