FDT 3.2 beta was just released this morning, including support for MXML and Flash CS4.

If you use FDT, this looks to be a huge update. Here is the list with main new features from the FDT blog:

MXML Support
- Semantic Highlighting
- Templates
- Wizard for MXML Classes
- Flex-Builder Project Importer
- Attribute Auto Completion
- Quick Outline
- Type Hierarchy
- Hyperlinks (F3)

Further AS-Support
- Improved SWC-Parser
- Improved Auto Completion (constructor)
- CS4 Help Importer

You can find instructions for updating to the beta here.