router.rb

# Extend the Request class to give
# a useful method to be able to route
# to subdomains
module Merb
  class Request
    def subdomain
      return nil if subdomains.first == "www"
      return subdomains.first
    end
  end
end

Merb::Router.prepare do |r|
  match(:subdomain => /(.+)/).to(:username => ":subdomain[1]") do
      # Everything within this block is sent :username => "mysubdomain"
      resources :posts
  end
end