The WCF REST Starter Kit Preview 2 provides several classes for managing traditional query string and form input to RESTful services:  HttpQueryString, HttpUrlEncodedForm, and HttpMimeMultipartForm.  These make passing data into RESTful services much easier.

Here's an example using HttpQueryString:

HttpClient http = new HttpClient("");
http.TransportSettings.Credentials =
    new NetworkCredential(username, password);
HttpResponseMessage resp = null;
// add query string variables
HttpQueryString vars = new HttpQueryString();
vars.Add("id", screenname);
vars.Add("count", count);

resp = http.Get(new Uri("user_timeline.xml", UriKind.Relative), vars);
And here's another example using HttpUrlEncodedForm:
HttpUrlEncodedForm form = new HttpUrlEncodedForm();
form.Add("status", status);

resp = http.Post("update.xml", form.CreateHttpContent());

These classes take care of properly encoding the input values, and the HttpUrlEncodedForm sets the appropriate content-type header for a form submission (application/x-www-form-urlencoded). You can also use HttpMimeMultipartForm when you need to generate multi-part MIME messages (to simulate a file upload etc). Check out this screencast to see these new classes in action.