Actually, several of the popular RTMP servers support username/password authentication. For example:
Flash Media Server http://help.adobe.com/en_US/FlashMediaL ... -7fe0.html
Wowzahttp://www.wowza.com/forums/content.php ... n-and-more
Without authentication, anyone that knows the RTMP URL to use can stream to that resource, which isn't very secure. That's why a lot of media server administrators require RTMP authentication. And that's why Flash Media Live Encoder (FMLE) prompts for a username and password when connecting to an RTMP resource that requires authentication.
Currently with XSplit, you're out of luck if you need to stream to an RTMP server that requires a username and password.
There's also a topic for this in the Requests from before the forums went up section. Seems like a pretty essential feature, so it's kind of surprising that this hasn't been added yet ...