Having the same problems. Five PCs one with both Vista and Win7 - so actually six (two WINXP x86, one Vista X86, three WIN7 X64). All, except for one WINXP PC will not show movie trailers. The one WINXP PC has consistently shown trailers. On all the PCs when I first installed QuickTimethe first test that I performed was to play a movie trailer - so I know that all PCs have, in the past, successfully showed trailers. I have tried all the suggestions in this thread, and none have worked. I have tied accessing via a VPN (which forces changes in DNS and other params) without success. Flushed DNS and Route, rebooted the cable modem and router, uninstalled/reinstalled JAVA and QuickTime, used IE 8/9 and Firefox, and even restored one WIN7 X64 C: partition to an off-the-disc clean config, installedQT/JAVA, and still cannot play movie trailers.
One other problem is that under the top row of movies (Beginners and Pirate) there is a menu bar with selections such as "Just Added","Exclusive", etc. If one of those is clicked I get "We're sorry, The Apple.com Trailers Application has encountered an error. The team has been notified and we're looking into it. Unexpected error."
Given all the above I conclude that this is an Apple problem, or at least a problem for which Apple should be providing a work-around until the cause can be rectified. For users, this just an inconvenience. But for the advertisers that are trying to get their trailers before a movie audience, it is significant Apple failure, regardless of the source.