Contrary to what we were told out at MacWorld by Apple Reps, I have successfuly installed 9.1 on OS X Public Beta 1. How I did it is as follows:
First I used a iBook 300 128mb ram, then I installed 9.1, then OS X PB1. Then I ran the classic app and everything worked just fine, the classic OS started up, and I ran a few basic classic apps just to make sure, ie: MS Word 98, and 4sight.
This is how I was able to do it, I encounterd no problems, but have not attempted to perform this act again due to lack of time on my part. Hope this helps someone!