Okay, so there's a built-in Progress HUD that looks cool. But it's not documented. So I found one at:
http://github.com/jdg/MBProgressHUD and decided to port it to monotouch.

To use: (I recommend keeping a reference to the object elsewhere in your code, so you can dispose of it properly)

1	var hud = new MBProgressHUD (this.View.Window);
2	hud.Mode = MBProgressHUDMode.Indeterminate;
3	hud.TitleText = "Loading";
4	hud.DetailText = "We'll be back shortly...";
5	this.View.Window.AddSubview(hud);
6	    hud.Show (true);

That's all! It's pretty snazzy, IMO. But the credit all goes to someone who isn't me. Anyhow, here's the source. Note that it's designed to sort of attach to a background process, and that part isn't quite done. However, the Show(animated) and Hide(animated) work just fine. That's enough for me.