And it always happens when you're using Safari, right? The Internet stops working, etc.
This is a known problem that existed sporadically on 10.4.2 and has just gotten a lot worse for some people with the Java Release 2 update. It's a conflict between launchd (whatever that is) and Java. I read a report on Macfixit.com this morning, and there's also a thread on the Apple.com forums. Additionally, I have filed a bug report with Apple (because I experienced this crash yesterday).
There are three ways to fix it if you're having serious problems.
1. When you start having the Safari problems, unplug your ethernet connection (if it's not wireless). This resets launchd.
2. Try running both Java utility programs in your Applications/Utilities folder. Reset or clear your Java caches.
3. If you must, archive-and-reinstall Tiger, but don't apply the Java Release 2 update afterwards.
At least, this sounds very similar to the issues I had with the machine *before* the Tiger upgrade. Now the hangs come much more often.