I just posted part 1 of the Ultimate Ruby Performance Test! I tested Rails vs. Rack vs. Sinatra serving ERB, Builder, HAML and plain HTML: http://www.ridingtheclutch.com/2009/07/13/the-ultimate-ruby-performance-test-part-1.html
Get whatever you can into Rack...almost as fast as Apache serving static HTML. Next is Sinatra at around half as fast as Rack. Last is Rails, but considering how much stuff it's doing in the background it's still pretty fast. Serving Rails through Thin is faster than Passenger, but all the options and customization Passenger has probably still makes it worth it.
In Part 2 I'm gonna put several instances of each behind Apache/nginx (these tests are only against 1 instance of each).
If anyone has any suggestions for testing or want to nitpick the results, let's hear it. ;)