LJ's listing of what a "getchallenge" call should look like reads:
POST /interface/xmlrpc HTTP/1.0
User-Agent: XMLRPC Client 1.0
So, trying to emulate this in Flash I was building the following XML to send:
Which threw a "failure during deserialization. Array can't be set to a null value" or something to that effect. After poking around and experimenting for a bit I came to the conclusion that the XML should be formatted like this:
And VOILA! It works.
I also found a very nice looking XML-RPC library written for ActionScript 2.0, which I plan to use rather than writing my own from scratch:
Hope that's helpful, if anyone else has been stuck on this... :)