The following links were posted on FlashCoders by Adobe’s David Mendels in the recent Flex vs. Flash thread. Just in case you haven’t been following along with the thread and are confused about how Flex fits into the big Flash picture, these are great reads.
The first article has been around since Flex 1 but it’s the exact article I was looking for to explain what Flex is and how it can be used from the viewpoint of someone only familiar with Flash:
http://www.adobe.com/devnet/flex/articles/flash_perspective.html
More recent articles about Flex 2 and Flash (AS3):
http://www.adobe.com/newsletters/edge/may2006/section2.html
http://www.adobe.com/devnet/flash/articles/flex2_flash.html
Flex Builder 2 for Flash Programmers Breeze Presentation: