Flex doesn't see reliable enough for handling XML.
This basic XML file causes an error in flex. Depending on the contents of the XML, it may or may not be invalid...and its not the characters. If I put in one type of UTF8 characters, the XML is OK, if I put in another type, its also OK. If I however put in both types...then the XML fails.
So in summary, good + good = bad.
Multiple browsers, multiple versions of Flash (latest, as well as a couple prior releases), multiple OS's.
XML headers indicate the content type correctly: Content-Type: text/xml;charset=utf-8
I worked around the bad XML handling in Flex by Base64 encoding my data, then Base64 decoding it in Flex. I used com.dynamicflash.util.Base64 to do the decoding.
Its a shame though...wasting extra CPU and making the flash slower just because it can't handle XML reliably.