Tweetcongress.org looks like a good resource. It's only a matter of time before non-profits and activist organizations start using twitter for "action alerts."
This might be a start. If constituents can get direct access to their representatives and are able to influence them, then grassroots movements may be able to influence the congress. The question is how much of the communication is one way from the representative to the constituents. If all that is happening is the reps using Twitter as an one-way mass communication tool, then we will remain where we are.