routes.rb

ActionController::Routing::Routes.draw do |map|

map.with_options :conditions => { :host => /^blog\.foo.com$/} do | blog |
  blog.connect '', :controller => 'blog', :action => 'index', :conditions => {:method => :get}
  blog.connect '/post/:id', :controller=>'post', :action=>'show', :conditions => {:method => :get}
  ... more blog stuff
end

map.with_options :conditions => { :host => /^foo.com$/} do | foo |
  foo.connect '', :controller => 'foo_home', :action => 'index', :conditions => {:method => :get}
  foo.connect 'why', :controller => 'foo_about', :action => 'index', :conditions => {:method => :get}
  foo.contact 'contact', :controller => 'foo_contact', :action => 'index', :conditions => {:method => :get}
  ... more corporate site stuff
end

end