Surprisingly, Sinatra was a major part of the first lab in the Rails 3 Deep Dive. The idea was to understand how to use Rack with middleware and Rails, using Sinatra as an easy introduction. I love to learn new stuff but, if you want to teach Rack this way, please give us a heads-up so that ignorance of Sinatra won't slow us down.
The takeaway: Rails 3 uses Rack much more than Rails 2. A lot of Rails filters may or have become Rack middleware.