I filed this bug this morning and thought it deserves a post as it took me quite a while to figure out how to do this. Now I know you won't necessarily be migrating for Flex 2 SDK to Flex 3 but you might and if you do please read on.

Here is the official bug. In a nutshell the problem is when you compile a project using the FDT Application features for AS3 projects with the Flex 2 SDK. I then migrated to the Flex 3 Beta SDK and the project was still compiling using the Flex 2 SDK. WTF?

It seriously took me like a while to figure this out. I couldn't just remove Flex 2 as I knew a few other projects was using it and working fine. I wasn't about to risk spending hours trying to solve multiple "unresolves" errors. So eventually I found the solution and it was when defining teh compiler settings under the FDT Application properties, the dropdown doesn't make the actual selection but rather the radio button does. If you need me to show yo specifically then contact me and I will send you some screen shots.

OS: Windows
Eclipse Version and Build ID: 3.3.0 I20070625-1500
FDT Feature: 3.0.3.1002
Java Version: jre1.5.0_06

I added the Flex 3 Beta SDK while having the Flex 2 SDK set as my Core library and FDT3 Application Flex SDK. By using Fast Add and adding the Flex 3 library and setting the project FDT parser settings to use, this only works if you do not have an existing FDT Application set-up.

By having an FDT Application setup that used CS3 or Flex 2 SDK and then changing it under Run->Open Run Dialog->FDT3 Application->SDKs using the dropdown selector doesn't actually work.

You have to click on the 'Use default SDK' radio button after using the dropdown for this change to take effect. If you compile your project without doing this the previous SDK will be used.

I know this as i can see what config file is loaded for the compiler and after the Flex3 SDK setup it was still using the flex2 config file.