I have just started to check out MODx as a CMS for Flex or Flash.
What is promising is the MODx "Ditto" snippet, which allows complex custom queries to the CMS

Here is the beginning...

http://get.adobe.com/air/
Download the attached AIR file (see ModXLinkup.air_.zip attachment to this article)
Run the AIR app and compare to the MODx page below:
http://lab.think-a-doo.net/modx/index.php?id=2