I just came across another great write-up on Adobe.com: Flex Quickstart Guide for PHP Developers. I've been using PHP for probably four or so years now, and this article really helped to drive some key points home about Flex. It shows you PHP and MXML/AS3 code side-by-side to clearly demonstrate key web application needs such as submitting a form with GET and POST, uploading a file to PHP, and populating a Flex DataGrid with XML produced from PHP.
It is nice to see Adobe tailoring to the web development group with Flex, and not just the enterprise Java and C/C# developers. Check out the article here.