Just posted the code that I am using for an iOS project of mine that handles all the Web Api calls.  

https://github.com/dsandor/WebApiClient/tree/master/src/Monotouch

Example of use:

WebApiClient.Post<ServiceResponse<User>>(string.Format ({0}Auth/Login?emailAddress={1}&password={2}
  , AppDelegate.Current.ApiBase, email.Value, password.Value), , )
.ContinueWith ( (t) =>

         {
            this.InvokeOnMainThread(() => { loadingView.Hide (); });
                            
            var result = t.Result;
                            
            if (result != null && result.IsSuccess && result.Payload != null)
            {
               this.InvokeOnMainThread(() => 
                {
                  AppDelegate.Current.UserInfo = result.Payload;
                  MoveToHomeView();
                                        
                 });
             }
             else
             {
                this.InvokeOnMainThread(() => {
                    new UIAlertView(Invalid login, Invalid username or password., null, OK).Show();
                    });
             }
     });