Here's a few useful posts concerning Authentication of HTTP requests.
In Flex (with HTTP Service class): http://www.geekzguru.net/howto-add-basic-authentication-header-to-httpservice/
In Flex (with URLLoader class): http://www.abdulqabiz.com/blog/archives/flash_and_actionscript/ http_authentica.php
and in general, using Flash to interface with RESTful APIs: http://dynamicflash.com/2008/11/ the-trouble-with-flash-and-rest/comment-page-1/
All very interesting to me currently - sometime I'll get down to writing a bit more about what I'm doing...