LJ's listing of what a "getchallenge" call should look like reads:
POST /interface/xmlrpc HTTP/1.0
User-Agent: XMLRPC Client 1.0
Host: www.livejournal.com
Content-Type: text/xml
Content-Length: 193
<?xml version="1.0"?>
<methodCall>
<methodName>LJ.XMLRPC.getchallenge</meth
<params>
<param>
</param>
</params>
</methodCall>
So, trying to emulate this in Flash I was building the following XML to send:
<?xml version="1.0"?><methodCall><methodName>L
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:
<?xml version="1.0"?><methodCall><methodName>L
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:
http://xmlrpcflash.mattism.com/
Hope that's helpful, if anyone else has been stuck on this... :)