I wrote the following code to put a point on a map on the iPhone.  It works pretty well.  Basically, I draw a map, then I inherit from the MKAnnotation object and create a new constructor, go out to geocoder and get a lat lon to senter the map on, and finally, I put a point in the center of the map.  I've got to thank Craig Dunn for the inspiration of inheriting from the MKAnnotation object.

using System;

using System.Collections.Generic;using System.Linq;using MonoTouch.Foundation;using MonoTouch.UIKit;using MonoTouch.MapKit;using MonoTouch.CoreLocation; namespace Json{public class Application{static void Main (string[] args){UIApplication.Main (args);}} // The name AppDelegate is referenced in the MainWindow.xib file.public partial class AppDelegate : UIApplicationDelegate{// This method is invoked when the application has loaded its // UI and is ready to runpublic override bool FinishedLaunching (UIApplication app, NSDictionary options){// If you have defined a view, add it here:// window.AddSubview (navigationController.View);window.MakeKeyAndVisible (); MapVw.ZoomEnabled = true;MapVw.UserInteractionEnabled = true;MapVw.ScrollEnabled = true;MapVw.ShowsUserLocation = true; btnMap.TouchDown += BtnMapTouchDown; LocationTF.EditingDidEnd += delegate(object sender, EventArgs e) {       UITextField utf = sender as UITextField;       Console.WriteLine("EditingDidEnd is finished.");       utf.ResignFirstResponder();}; LocationTF.Ended += delegate(object sender, EventArgs e) {}; LocationTF.EditingDidEndOnExit += delegate(object sender, EventArgse) {}; return true;} void BtnMapTouchDown (object sender, EventArgs e){GeoCode gCode = new GeoCode();string location = LocationTF.Text;double lat = 0.0, lon = 0.0, radius = 0.0;Location.ResignFirstResponder();gCode.GetLatLon(location, ref lat, ref lon, ref radius);Console.WriteLine("Lat: " + lat.ToString() + " Lon: " + lon.ToString());MapVw.Region = new MKCoordinateRegion(new CLLocationCoordinate2D(lat, lon),                                      new MKCoordinateSpan(.5, .5));MapVw.ZoomEnabled = true;MapVw.UserInteractionEnabled = true;MapVw.ScrollEnabled = true;btnMap.TouchDown += BtnMapTouchDown;gCode = null;} // This method is required in iPhoneOS 3.0public override void OnActivated (UIApplication application){}   }

}

The  MKAnnotuation object is inherited like this:

using System;using MonoTouch.UIKit;using MonoTouch.MapKit;using MonoTouch.CoreLocation;namespace Json{// concept borrowed from Craig Dunn’s blog.   public class ObjAnnotation : MKAnnotation {           private CLLocationCoordinate2D _coordinate;    private string _title, _subtitle;//getters must be overridden to return necessary data.    public override CLLocationCoordinate2D Coordinate {       get { return _coordinate; }    }//title and subtitle are readonly, thus no setter.    public override string Title {       get { return _title; }    }    public override string Subtitle {       get { return _subtitle; }    }    /// <summary>    /// Need this constructor to set the fields, since the public    /// interface of this class is all READ-ONLY    /// <summary>    public ObjAnnotation (CLLocationCoordinate2D Coordinate,                             string Title, string SubTitle) : base()    {       _coordinate=Coordinate;       _title=Title;       _subtitle=SubTitle;    }} }

The result is this:

 

Want to know more about developing with the iPhone?  Check out my Wrox Blox eBook on developing applications with MonoTouch for the iPhone/iPod touch for .NET/C# developers.