MonoTouch/iOS number pad keyboard needs a UIToolbar to include a button to resign the first responder of a UITextField because it doesn't include any built in way. This is a custom sub-classed UITextField which given a keyboard type that is a number pad will format a toolbar with a done button.

MonoTouch UITextField with DoneButton for NumberPad

using System;
using MonoTouch.UIKit;
using System.Drawing;

public class CustomTextField : UITextField
{
	public CustomTextField (UIKeyboardType keyboardType) {
		// iPad doesn't really need it.
		if (keyboardType == UIKeyboardType.NumberPad && DeviceHelper.IsIPhone()) {
			UIToolbar numPadToolBar;
			numPadToolBar = new UIToolbar(new RectangleF(0,0,320,27));
			numPadToolBar.BarStyle = UIBarStyle.Black;
			UIBarButtonItem doneButtonItem = new UIBarButtonItem ("Done", UIBarButtonItemStyle.Done, null);
			doneButtonItem.Clicked += delegate(object sender, EventArgs e) {
				this.ResignFirstResponder (); 
			};
			
			// add in any other buttons you might have created such as "Revert"
			UIBarButtonItem[] numPadToolbarButtons = new UIBarButtonItem[] {doneButtonItem};
			numPadToolBar.SetItems (numPadToolbarButtons, false);
			
			// wire it up
			this.InputAccessoryView = numPadToolBar;
		}
	}
}