The biggest change with Rails 2.1 that impacts this beta of the book is that the way migrations are named, processed, and stored in the database changed. There is a somewhat more minor change in that some of the contents of the generated templates changed slightly, but not in any functional way. Future betas will cover some of the new features of “Rails 2.1”:http://weblog.rubyonrails.org/2008/6/1/rails-2-1-time-zones-dirty-caching-gem-dependencies-caching-etc, but you simply won’t be able to take advantage of these features just yet.
Overall, I don’t think you will have any major problems (I developed betas 1.0 through 1.3 using Rails 2.0.2), but if you get stuck feel free to ask questions here.
From what I am hearing, Rails 2.2 will beta in early September. I don’t believe that a final release date has been set.