hos21, something that will cause that is QT Add-ons. I'll use IE8-Internet Explorer 8 browser as an example. If you go to the Tools menu-Manage Add-ons. There should be:
iTunesDetector Class Apple Inc.
QuickTime Object Apple Inc.
QuickTimeCheck Class Apple Inc.
If they are showing as being disabled it will produce the message your getting to download and install QT. By right clicking on them you can choose to enable. Which should then let the computer play the trailers.
For what it's worth - I had Firefox 3.5.5 and QuickTime 7.5.5 installed and I could watch the trailers on the Apple site. I updated both to Firefox 4.0 and QuickTime 7.6.9 and the videos would no longer load. The site itself responded fine, I could watch the trailers with IE 8 and QuickTime movies I had on my computer played properly. I loaded a backup of my system with the old settings and everything works again. So either there's a problem with Firefox 4.0 or QuickTime 7.6.9 or the two together with my specific configuration. I have a hard time believing that the problem is unique to my system.