<p>I'm trying to consume a SOAP webservice from an Adobe Flex 3 application, but the server tell me "Invalid SOAP Envelope. SOAP Body does not contain a message nor a fault". I already wrote other test clients (with both Delphi and C#) and I'm sure it's all ok on the server side, so I need to examine the SOAP envelope Flex is sending out to the server. How to do that? I think it should be some event to listen (in the BaseSys class?) to get the envelope before it will be sent.</p>

The easiest way is to run a proxy. Paros is an easy one, written in Java, and therefore multi-platform by nature :
http://www.parosproxy.org/index.shtml
Also, if you do not use it already, you should install firebug :
https://addons.mozilla.org/fr/firefox/addon/1843
The network monitoring tab should fit your needs.