Here's the problem I had, and how I solved it. First, I swapped my standard Apple DVD drive for brand new DVD dual-layer burner. Then I changed my password whilst under the influence (don't do that -- it'll end in tears). I forgot the password after I sobered up, and at the same time, found that the new drive wouldn't read the Tiger install DVD.
So I booted using the Panther install CD, which seemed to work just fine. I selected the Reset Password option. It found my users (root and main), and allowed me to reset their passwords. Apart from some Keychain issues (which I am fixing now), I am once again able to install software, update the system, etc. So, it seems that a Panther disk will allow you to change passwords for users with a Tiger update. Hope this saves someone from having to go through the same 'tearing hair out' process that I have. Remember, back up your data before trying something like this. In my case, Tiger was installed over Panther, and I'm unable to test on a clean Tiger install.
[robg adds: Please don't comment that this is a security hole ... the Reset Password function is right there on the install discs, ready to be used on any machine. If someone has their hands on a Tiger install disc, they could just as easily have a Panther install disc. This hint will simply help someone who has a non-functional Tiger CD and a forgotten password.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050819203033740