The Flex framework can be used without the IDE with just the command-line tools (but it's probably not fun).
The IDE, called Flex Builder 2 now (previously code-named Zorn), can either be installed as a standalone executable (124 MB) or as an Eclipse plug-in (38.1 MB). The charting components are available as a separate download (22 MB).
The new Flash Player 8.5 is required for Flex 2 applications to run. The latest player contains an implementation of the new ActionScript 3 programming language and a new virtual machine called AVM2.
This is alpha quality software. Expect to encounter lots of bugs and feel free to report them through appropriate channels. We're releasing this software just to give you a headstart on Flex 2 development and to get early feedback from the community.
If you build something cool in MXML and AS3, feel free to blog about it!
There's a wiki on the Labs site where you can share samples and other useful content.
As I said earlier, if you're at MAX, you have the option of walking up to me and having all of the bits copied onto your system instead of going through the huge 100+ MB download. You'll find me at the Community Lounge or hanging around with randompeopleyouprobablyknow.
So, all set? ;) I hereby declare MXML mainstream. :) And just to end it on a fancy note: RIA development will never be the same.