I seem to be getting almost the correct result with the following. Not exactly right, but close. It also attempts to put the cursor on the point even if the point is not visible on-screen, which may result in a cursor that is way off the visible screen.
Point3d pt = new Point3d(100, 200, 0);
Autodesk.AutoCAD.ApplicationServices.Document doc = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;
int viewPortNumber = Convert.ToInt32(Autodesk.AutoCAD.ApplicationServices.Application.GetSystemVariable("cvport").ToString());
System.Drawing.Point ptOffset = ed.PointToScreen(pt, viewPortNumber);
System.Drawing.Point windowLocation = doc.Window.Location;
Cursor.Position = new System.Drawing.Point(windowLocation.X + ptOffset.X, windowLocation.Y + ptOffset.Y);