During FITC Toronto, Richard Galvan demonstrated some of what's new in Flash Professional CS5. He started with what's easily the killer feature in CS5, the Text Layout Framework. TLF adds literally dozens of new properties and settings for controlling your typography.
You can create multiple columns and set text to flow automatically between them. On your stage you see when the text is overflowing a field and you can drag and create links between textfields for the overflow to spill into. Support for character sets have been improved, including left to right and vertical reading. They've even added in properties required specifically for certain character sets, like Arabic. Font embedding is improved and much easier. All this is managed by a TLF swc that Flash automatically checks online for updated versions.
In CS4, Inverse Kinematics was added to allow the creation of bone structures between objects. In CS5 Adobe has added an underlying physics engine that is enabling some new IK features. At this point there is no access to this physics engine other than through the IK tools, but Galvan suggested this will enable additional features in the future (I'm thinking back to some gravity demos that were shown during the Sneak Peaks at MAX last fall). The new IK settings you have available are spring, strength and dampening. These create some really wonderful new movements when building characters with IK bones.
One really big addition is the move to using the XFL file format in place of the FLA. Being able to open and edit this xml based format makes it easy to allow multiple people to work on a Flash project by giving them distinct access to the assets that make up a project. You can even edit many of the other properties that make up your project. The XFL spec will be released to enable 3rd party developers to create tools to work with XFL.
There are many reasons why Flash CS5 is fantastic. Check it out!