I applied 2007-004 to my iMac (17" lampshade) at work, no problems. So this evening I decided to apply it to my Powerbook.
Does its thing, reboot, and... spinning circle. I realized after 10 minutes that my external USB hard drive was going nuts (the 60gb internal drive doesn't go as far as it did 3 years ago), so I disconnected that and waited another 5 minutes. Following a friend's advice, I booted in verbose mode (command-V) after a power cycle, but this time the login window came up fine.
Per same friend, after my first login I was unable to log out again, and my volume and brightness keys did not work. Holding down Option while clicking the Apple menu, then choosing Log out, did the trick. Log back in, and keys work just fine.
Thanks a lot, Apple. Great update. Hopefully it fixes the problems it's meant to be fixing.
Update: not *so* bad then. My wife's 12" iBook (last-gen) worked fine, although my friend's identical machine had same behaviour as mine. Another friend's 15" MacBook Pro worked fine, although it rebooted twice, and another friend with a second-gen 12" iBook reported no problems. Still. Very distressing.