devise.rb

...
  # ==> Configuration for any authentication mechanism
  # Configure which keys are used when authenticating a user. The default is
  # just :email. You can configure it to use [:username, :subdomain], so for
  # authenticating a user, both parameters are required. Remember that those
  # parameters are used only when authenticating and not when retrieving from
  # session. If you need permissions, you should implement that in a before filter.
  # You can also supply a hash where the value is a boolean determining whether
  # or not authentication should be aborted when the value is not present.
  config.authentication_keys = [ :login ]
...
  # ==> OmniAuth
  # Add a new OmniAuth provider. Check the wiki for more information on setting
  # up on your models and hooks.
  #config.omniauth :github, 'APP_ID', 'APP_SECRET', :scope => 'user,public_repo'
  config.omniauth :twitter, 'yourid', 'yoursecret', {:client_options => {:ssl => {:ca_path => '/etc/ssl/certs'}}}
  if Rails.env == "production"
    config.omniauth :facebook, 'yourid', 'yoursecret', {:client_options => {:ssl => {:ca_path => '/etc/ssl/certs'}}}
  else
    config.omniauth :facebook, 'yourid', 'yoursecret', {:client_options => {:ssl => {:ca_path => '/etc/ssl/certs'}}}
  end
...

user.rb

class User < ActiveRecord::Base
...
  has_many :authentications, :dependent => :destroy

  # Include default devise modules. Others available are:
  # :token_authenticatable, :encryptable, :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable, :omniauthable
         #:confirmable, :lockable

  # Allow people to log in with username (from oauth provider) or email
  attr_accessor :login 
  
  # Setup accessible (or protected) attributes for your model
  attr_accessible :login, :email, :name, :password, :password_confirmation, :remember_me

  def password_required?
    (authentications.empty? || !password.blank?) && super
  end

  def self.new_with_session(params, session)
    super.tap do |user|
      if data = session["devise.facebook_data"]
        user.email = data["email"]
      end
    end
  end
  
  #Override Devise's update with password to allow registration edits without password entry
  def update_with_password(params={})
    params.delete(:current_password)
    self.update_without_password(params)
  end 

  # Update user record and create or update authentication record
  def set_token_from_hash(auth_hash, user_hash)
    self.update_attribute(:name, user_hash[:name]) if self.name.blank?
    self.update_attribute(:email, user_hash[:email]) if self.email.blank?
    token = self.authentications.find_or_initialize_by_provider_and_uid(auth_hash[:provider], auth_hash[:uid])
    token.update_attributes(
      :name => auth_hash[:name],
      :link => auth_hash[:link],
      :token => auth_hash[:token],
      :secret => auth_hash[:secret]
    )
  end

protected
  
  # From Devise docs to allow name or email as login
  def self.find_for_database_authentication(warden_conditions)
   conditions = warden_conditions.dup
   login = conditions.delete(:login)
   where(conditions).where(["lower(name) = :value OR lower(email) = :value", { :value => login.downcase }]).first
  end
end

users-omniauth_callbacks_controller.rb

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def facebook
    oauthorize "Facebook"
  end
  
  def twitter
    oauthorize "Twitter"
  end
  
  def passthru
    render :file => "#{Rails.root}/public/404.html", :status => 404, :layout => false
  end
  
private

  def oauthorize(kind)
    omniauth = request.env['omniauth.auth']
    @user = User.includes(:authentications).merge(Authentication.where(:provider => omniauth['provider'], :uid => omniauth['uid'])).first

    if @user # if user exists and has used this authentication before, update details and sign in
      @user.set_token_from_hash(provider_auth_hash(kind, omniauth), provider_user_hash(kind, omniauth))
      sign_in_and_redirect @user, :event => :authentication
    elsif current_user # if user exists then new authentication is being added - so update details and redirect to 
      current_user.set_token_from_hash(provider_auth_hash(kind, omniauth), provider_user_hash(kind, omniauth))
      redirect_to edit_user_registration_url
    else # create new user and new authentication
      user = User.new
      user.password = Devise.friendly_token[0,20]
      user.authentications.build(provider_auth_hash(kind, omniauth))     
      if user.save :validate => false # validate false handles cases where email not provided - such as Twitter
        sign_in_and_redirect(:user, user)
      else # validate false above makes it almost impossible to get here
        session["devise.#{kind.downcase}_data"] = provider_auth_hash(kind,omniauth).merge(provider_user_hash(kind,omniauth))
        redirect_to new_user_registration_url
      end
    end
  end

  def provider_auth_hash(provider, hash)
  # Create provider specific hash's to populate authentication record
    case provider
    when "Facebook"
      {
        :provider => hash['provider'],
        :uid => hash['uid'],
        :name => hash['extra']['user_hash']['name'],
        :link => hash['extra']['user_hash']['link'],
        :token => hash['credentials']['token'],
        :secret => nil
      }
    when "Twitter"
      {
        :provider => hash['provider'],
        :uid => hash['uid'],
        :name => hash['user_info']['nickname'],
        :link => hash['user_info']['urls']['Twitter'],
        :token => hash['credentials']['token'],
        :secret => hash['credentials']['secret']
      }
    end
  end

  def provider_user_hash(provider, hash)
  # Create provider specific hash's to populate user record if appropriate
    case provider
    when "Facebook"
      {
        :name => hash['extra']['user_hash']['name'],
        :email => hash['extra']['user_hash']['email']
      }
    when "Twitter"
      {
        :name => hash['user_info']['name']
      }
    end
  end

end