I agree with the recommendation of having an earlier revision of the OS handy, but you don't necessarily have to have another Mac for this. I split my hard drive into multiple partitions, with which you can install different versions of the OS. Hard drives are certainly large (and cheap) enough to do this.
Another consideration is to keep these spare versions on an external FW drive. My friend has done this in the past, and it has come in handy. 10.4.2 itself was problematic for us on our campus because of some weird conflict with our Exchange server and the version of Mail that come with it. Being able to fall back to 10.4.1 saved me this time.
Maybe Apple should include a rollback feature.