Found this at http://blog.simongregory.com/09/as3-autocompletion-in-textmate/ . I'll admit it's not to the level of Flex Builder, but it's great to see actual autocomplete working in TextMate for AS 3. Once installed, hit option-esc to bring up the autocomplete suggestion list. If you choose a function it will give you the the expected parameters:

PLAIN TEXT
Actionscript:
  1. stage.addEventListener(type:String,listener:Function,useCapture:Boolean=false,priority:int=0,useWeakReference:Boolean=false);

This is the flaky part, because the idea is that you can tab through the parameters to enter your values. But say if you wanted to listen for MouseEvent.CLICK, and don't have the MouseEvent class imported alread. You can easily import it by starting to type M.. and hitting apple-shift-I to import it (then again using autocomplete to choose 'CLICK'), but then you can't tab over to the rest of the parameters. I'm going to dig around to see if there's a workaround.

Other than that, seems to work fine with custom classes, i can autocomplete methods and function just fine. Mix this the other tab triggers in the bundle and you really start to feel a big speed increase when busting out the code.

By the way, apparently there is some confusion because there is an older AS3 bundle. I wound up removing the one i already had installed, and using the GetBundles bundle (a bundle that acts like a package manager) to install the newer version, then the autocomplete worked properly.