This week Jetbrains released IntelliJ IDEA 9.0 which has flex support. We have checked it out, and can already conclude that this is a very strong contender when it comes to Actionscript 3 editing. Superior refactoring, code analysis, flexible source code formatting, plenty of smart code generation functionality, (much) easier to set up than FDT or flexbuilder, quick and superb navigation, .... We are great FDT fans, mainly for the speed advantage when comparing to flexbuilder, but when jetbrains gets rid of some of the relatively harmless infancy ".0" problems we will probably switch. Another interested developer told that if you can prove you are a switcher who comes from another IDE, they'll give you a 25% price cut.
Just like FDT, IDEA does not have a "built in" profiler which is a still an interesting feature in flexbuilder, but I hope to see profilers in other IDE's over the next few years. [Although Joa Ebert tells me: "hey, just roll your own with flash.sampler.*!"]