When you try to build a project with the Flex 2.0.1 SDK using the FlexBuilder Gumbo MAX Preview build, you'll encounter an error indicating that the flex-compiler-oem.jar is missing from the classpath. I've logged this as the following issue: https://bugs.adobe.com/jira/browse/FB-15688. The issue was resolved as Not a Bug with the comment Flex Builder 4 will not support Flex SDK 2.
I'll repeat that. On a separate line. In Bold. Pay attention.
FlexBuilder 4 will not support Flex SDK 2.
Of course, ANT scripts and command-line compiles will continue to work. However, losing IDE support should be a pretty compelling reason to look into the upgrade process.
Flex 3.0 was released February 25, 2008. Since release the SDK has been updated to Flex 3.1 on August 15 and Flex 3.2 on October 29.
To learn more about the Flex SDK and download new versions, visit http://opensource.adobe.com/wiki/display/flexsdk/Flex+SDK.
For those of you looking to upgrade from Flex 2 to Flex 3, check out the compatibility issues in the Flex 3 release notes.
And now, I leave you with Monty Python's "Bring Out Your Dead". Flex 2.0.1, you'll be stone dead in a moment.