Using a MS ISA proxy server, with authentication required, No Quicktime movies will load when they are embedded in a web page. This only happens in Safari... Firefox, IE, Mozilla work just fine. In Safari, the Quicktime symbol (Q) come sup with a "?" on top of it. This has been the case ever since 10.4 came out. (Actually, since 10.4.2 since NO pages loaded through an ISA proxy in 10.4.0 and 10.4.1).
It can be worked around by using a local proxy server like Authoxy or Squidman.
It seems like there is no one around with an authenticating MS ISA proxy around to confirm this issue :(
However, I did a quick Google search, and the problem appears to be quite well known.
The QuickTime plugin bypasses the browser for network access, so it's very likely to be a problem in QuickTime, not in WebKit. The reason this works in other browsers could be that they use a different version of QuickTime plugin (WebKit has its own plugin API).
Please report this problem to Apple via <http://bugreport.apple.com> (free registration required). Given how much coverage it has got on the Web, it's likely that someone has already reported it, but one can never know for sure. Marking INVALID as a non-WebKit issue.