I’ve been hearing a number of reports that the Mac OS X 10.4.9 update causes significant issues with some Audio Unit plug-ins, including those from popular developers Audio Damage and Ohmforce. (See Analog Industries blog for a comment thread and reports on Audio Damage; there’s also discussion on the Core Audio developer list which I hope will yield some revelations. Update: Note that Audio Damage is having only development issues; their plug-ins will work just fine if you’re a user.) This update apparently installs yet another version of the AU validator, which could be one clue. Apple has also made significant changes to QuickTime in recent releases, which can also cause issues with audio software in some cases.
Your best bet: don’t install the updates the Software Update recommends unless you can easily go back to the previous version. And hold off on 10.4.9 for now.
Users and developers are understandably angry with Apple: rather than bundle updates together, Apple has been making lots of minor changes in successive updates, and developers too often find about changes after things break. That’s especially problematic when the changes are being made to an API like AU — the whole point is supposed to be providing a solid platform for developers. Apple either needs to adjust the way it rolls out updates or communicate better with its developers. Be aware, as well, that the upcoming 10.5 upgrade is an enormous release; I expect you may want to wait for updates to catch up with that when it comes out (when that is, no one knows).
Now, no Windows vs. Mac arguments, please: the bottom line is, developers are having a rougher time than they should on both platforms. (On the Vista side, what I’ve heard from developers is that they had the same difficulties the rest of us did — it was tough to get test systems up and running because of new compatibility and driver issues, so it was hard to test in advance.) Updates and the resulting bugs are a necessary evil, but there’s clearly room for improvement on both the Apple and Microsoft side that could result in a healthier, happier platform for everyone.