> Is it RESTful to add GET parameters to
> a collection in order to add
> functionality such as filtering,
> sorting, or pagination?

Yes, I think so, because the query string is also just a part of the uri. So the collection -- as a resource -- stays identifyable. One entity resource may also get identifyable by more than one uri, which means, that the way twitter solved this, can be considered as a reference to the "real uri" with query string.