"asunit.runner.BaseTestRunner.trace" works very well for asUnit+FD3.

1-Download & install asUnit http://www.asunit.org/

2-Add the asUnit framework to global classpath (found in wherevererYouInstalledAsUnit/framework/as25 OR as3 folder)

3-Create a new Project

4-set the Custom Trace Function option in the project/properties/compiler options to "asunit.runner.BaseTestRunner.trace"

5-Have a look at http://www.flashdevelop.org/community/viewtopic.php?t=529&highlight=asunit for the XUL UI button

There may be a bug there since the "Custom Trace Function" field is empty even a value is set.

Unfortunately I have no Idea about the XUL UI since it crashes instantly on my vista. I'm writing the whole cases myself.
Rolling Eyes

IMO it should be very easy to add a third "Unit Test" option to debug/release combo. Project templates may contain a unitTest Folder which XUL UI will create its tests and a Main.as as test's entry point. I checked the settings files but couldn't find anything anything moddable. I guess it's hardcoded so I'm asking it as a feature. I think FD3 release must have this feature.

Edit: I managed to run the XUL UI. A shortcut on the desktop which points to: "C:\Program Files\AsUnit\xulrunner\xulrunner.exe" -app ../xului/application.ini did the trick. However I found that the XUL UI does nothing fancy but finding the classes that extend TestCase and build the necessary test classes. Again not a big job to add to FD3

Edit2: I found these two very good tutorials about the asUnit FW:
Part One: http://timbeynart.com/docs/ASUnit_Part1.pdf
Part Two: http://timbeynart.com/docs/ASUnit_Part2.pdf

Async testing is done in a much simpler manner than I thought. You simply override the run method not to run the test. Instead you add super.run() to the async method of your testCase. However a seperate TestCase must be created for each async method.

I'd do a plugin but I'm leaving in a week for military duty and hope someone finds this info useful when I'm dealing with potatoes.