Two chapters of my new book (Practical Ruby Projects: Ideas for the Eclectic Programmer) are dedicated to building a simple turn based strategy game in Ruby along with a pretty RubyCocoa interface.

http://www.amazon.com/Practical-Ruby-Projects-Eclectic-Programmer/dp/159059911X/

I'm looking forward to trying out Ruby and SDL together at somepoint...

--Topher Cyll