Fabio Terracini of Dclick has released a document about Flex (MXML and ActionScript) coding conventions that he uses on a regular basis. His objective: provide a common and consistent language to help code comprehension between developers.

Unfortunately the document is out of date. it is almost over 2 years old and no updates have been done since February 2007.
The document can still be found here.
But some stuff is like I said outdated. Like the link to the Creating ASDoc Comments from the Adobe site. You can find that infomation here.
Another good web page to start with when you need coding guidelines is the Adobe Open Source Flex SDK Coding Conventions page.
There is also a good deal of information on Developer Documentation on the Adobe Open Source site .