In Ruby 1.9, or Ruby 1.8 w/ ActiveSupport, you can use the & operator, which coerces it's argument (in this case, a symbol) to a Proc:

    all_users.select(&:is_admin).map(&:username)

This is a common idiom with a fantastic writeup at http://weblog.raganwald.com/2008/02/1100inject.html