I was getting a "(no picture)" message in Safari windows in place of streaming content after upgrading to QuickTime 7.1. This makes the trailers site truly depressing, I'll tell you what! A quick search found this article on MacFixIt regarding some sound issues, and a reply by a user named Swift2 with a comment and a fix for the issue I was having.
It turns out that the latest version of VLC wants to install a web plug-in, and if you let it do so, it will cause this problem. To solve the problem, just remove VLC Plugin.plugin from /Library/Internet Plug-Ins, then relaunch your browser.
The only other glitch I noticed with the new version of QuickTime is that if you play an MPEG in an Inspector preview or in column view, it will pause a few seconds from the end and then jump to the last frame. Anyone else?
Mac OS X Hints