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?

