Jensen Harris has started blogging about the new Office "12" UI. Folks who attended the keynotes at PDC, got to see the first look at the new UI. From everyone I talked to, there is a lot of excitement and I heard over and over that folks can't wait to get their hands on Beta 1.
Jensen's blog should be a great read. He has a lot of knowledge around the evolution of the Office user interfaces and will probably mix in a lot of historical information on top of all the new stuff.
Also, Steven Sinofsky talks a bit about learning from the past in this blog post: http://blogs.msdn.com/techtalk/archive/2005/09/12/464152.aspx. It's a great read.