To Overlay or Not To Overlay

I can't find a clear explanation of  what is required for us to use for a new SDK when it is released.

1) Are we still supposed to do the overlay? Or is this no longer necessary?

2 )What is the difference between the Flex version and the non-flex version? If you are using Flash Builder but creating an ActionScript project, then are you supposed to use (if you are on a mac) air15_sdk_mac?  What if you then also have a Flex project? Are you then supposed to download AIRSDK_Compiler?  Or do you already have that?

Thanks.