I just talked to Dan Gillmor who installed the new Quicktime 7.0.1 upgrade and looks like it has broken the OS. He was running Tiger with a pretty vanilla install. Now it won't get past the end of the boot screen. This is a warning to anyone who is thinking about doing the upgrade. I would wait until there is more news.
Any information about this new upgrade would be greatly appreciated.
UPDATE: Dan's the only one I've talked to who has had this problem so it's probably an isolated incident. Sorry about the false alarm. Wanted to get word out just in case. It seems like it is safe to do the upgrade although I'm going to wait until I get home today.
UPDATE 2: Markoff calls this "Gillmor luck".