gistfile1.cs

twilio.SendSmsMessageAsync("5551111111", "5552222222", "Async! ", (msg) => {
    Dispatcher.BeginInvoke(() => { txtOutput.Text = msg.Sid; });
});

twilio.SendSmsMessageAsync("5551111111", "5552222222", "Async! ", smsSent);
public void smsSent(SmsMessage msg) {
    Dispatcher.BeginInvoke(() => { txtOutput.Text = msg.Sid; });
}

// or

twilio.SendSmsMessageAsync("5551111111", "5552222222", "Async!");
twilio.SendSmsMessageCompleted += smsCompleted;

public void smsCompleted(object sender, SmsCompletedEventArgs e) {
    Dispatcher.BeginInvoke(() => { txtOutput.Text = e.Message.Sid; });
}

In Rx, you would have...
IObservable o = twilio.SendSmsMessage("5551111111", "5552222222", "Async!");

And then the consumer would subscribe:
using(var sub = o.Subscribe(smsSent))
{
// Whatever...
}

The real advantage of Rx is composability, but I'm not entirely sure how that would be useful here...

That looks awesome! I think I need to dig into Rx.........