Well there are many cartesian rectangular coordinate system. Strictly speaking a Plate Carree projection is a cartesian rectangular coordinate system using longitude and latitude as X,Y.
if you are trying to go from the screen coordinates in pixel to the map coordinates in whatever system the map is defined IMapControl2.ToMapPoint will do that. If you are looking to go from a specific projected coordinate system (X/Y) to a geographic one (lon/lat), you can create a point, define it's projected coordinate system, project the point to a geographic coordinate system (of the same datum.)