Well, I think this is more a problem of some word
puzzling. At the beginning I was confused by that,
Flex is nothing more than a development framework
that produces a SWF file.
Well, AFAIK Flash
is only the authoring tool/IDE
by Adobe. But in general people speak about the Flash platform
meaning the flash player (virtual
machine) which executes SWF files.
Flex as a development framework can be used with
actionscript3.0 only - or you can use flex' markup
language called MXML (or a mix out of both)