How to get a WinForm synchronization context or schedule on a WinForm thread

Question

I have a winform application, and an observable set up like this:

Form form = new Form();
Label lb = new Label();
form.Controls.Add(lb);

Observable.Interval(TimeSpan.FromSeconds(1))
          .Subscribe(l => lb.Text = l.ToString());

Application.Run(form);

This doesn't work, since the l => lb.Text = l.ToString() will not be run on the main thread which created the form, but I cannot figure out how to make it run on this thread. I assume, that I should use IObservable.SubscribeOn which takes either an IScheduler or a SynchronizationContext, but I don't know how to get the synchronizationcontext of the main thread, and the only Schedulers I could find were the static properties of Scheduler, such as Scheduler.CurrentThread, Immediate, NewThread, TaskPool and ThreadPool, none of which worked.

My Rx version is 1.0.10621.