Ruby Central is now registered with Google as a mentor organization for the 2006 Summer of Code. There is a Ruby Central page listing the current set of project suggestions.
If you have a particular Ruby project you want to work on as a part of the Summer of Code, it appears that you will first have to get it recognized by Ruby Central (or another Ruby-focused mentoring organization, should they be added), and then "submit to Google proposals to work with particular mentoring organizations; Google routes to the mentor organization, and the proposals are ranked by mentors working with those organizations."
See here.
Read: Ruby Central and the Summer of Code