Rails2.3のRack統合でActionController::AbstractRequestがActionController::Requestに統合されたのが原因だった模様。下記のように書き換えて対処。 module OAuth::RequestProxy #:nodoc: all class ActionControllerRequest < OAuth::RequestProxy::Base # proxies ActionController::AbstractRequest if ActionController.const_defined?(:AbstractRequest) proxies ActionController::AbstractRequest else proxies ActionController::Request end # ... end