QuickTime streaming server supports authentication for people who connect with QuickTime player; however, when you connect with QuickTime X in Snow Leopard to a server that requires authentication, you will get an "Unauthorized" message and the media will not play.
Apple's new version of QuickTime player is rebuilt from the ground up instead of building off and maintaining the old technologies i that have been around since the Classic Mac OS. In doing this Apple has had to start over with many aspects of QuickTime player, and while there are a number of new features, some of the player's player's capabilities are limited. Unfortunately, one of those features is support for authentication in quicktime streams. QuickTime X cannot authenticate either to streaming servers or to the http streaming video support that Apple has advertised in QuickTime X.
If you need to authenticate either to a qtaccess file or Open Directory binding on a Quicktime server, you can only do so with QuickTime 7. Luckily, version 7 is available as an optional install on the Snow Leopard DVD, so be sure to install and try that version if you are having problems authenticating to a streaming server. For information on how to install QuickTime 7, see this Apple KB article: http://support.apple.com/kb/HT3678
As a regular user of QuickTime streaming server, I've seen a number of glitches and limitations in both the broadcaster and the player that I would very much like to see addressed by Apple. I hope the next update to QuickTime adds authentication support back.