# models/user.rb
with_options :if => :should_validate_password? do |user|
  user.validates_presence_of :password
  user.validates_confirmation_of :password
  user.validates_format_of :password, :with => /^[^\s]+$/
end

attr_accessor :updating_password

def should_validate_password?
  updating_password || new_record?
end

# routes.rb
map.with_options :controller => 'sessions' do |sessions|
  sessions.login 'login', :action => 'new'
  sessions.logout 'logout', :action => 'destroy'
end