I agree with Will.  Unfortunately, the Timer class does not appear to launch a separate thread but instead simply preempts the execution of the main thread.  I found this out while I was toying around with some code for genetic algorithms in AS3 and the UI locked up whenever the timer would fire.  Not only that, but if you are trying to run code that requires more calculations than what you allotted for your timer but at the same time requires prior results, you are pretty much screwed.  The only reason I'm sticking with flash is that I'm developing simple games that I sell to flash sites.  Otherwise, I'd probably dump it for the unity framework.