ESRI has a utility class for converting Lat Long from/to Web Mercator for use with bing/google maps.

They provide this functionality through a dll (ESRI.ArcGIS.Client.Bing) which requires Silverlight to be installed. Don’t have/want Silverlight on your web server, don’t have/want ArcObjects? Then nab this:

private void WebMercatorToGeographic(double mercatorX, double mercatorY, out double lat, out double lon)
{
if ((mercatorX < -20037508.3427892) || (mercatorX > 20037508.3427892))
{
throw new ArgumentException(”Point does not fall within a valid range of the mercator projection.”);
}
double x = mercatorX;
double y = mercatorY;
double num3 = x / 6378137.0;
double num4 = num3 * 57.295779513082323;
double num5 = Math.Floor((double)((num4 + 180.0) / 360.0));
double num6 = num4 - (num5 * 360.0);
double num7 = 1.5707963267948966 - (2.0 * Math.Atan(Math.Exp((-1.0 * y) / 6378137.0)));
lat = num6;
lon = num7*57.295779513082323;
}

private void GeographicToWebMercator(double lat, double lon, out double mercatorX, out double mercatorY)
{
if ((lat < -90.0) || (lon > 90.0))
{
throw new ArgumentException(”Point does not fall within a valid range of a geographic coordinate system.”);
}
double num = lat * 0.017453292519943295;
double x = 6378137.0 * num;
double a = lon * 0.017453292519943295;

mercatorX = x;
mercatorY = 3189068.5*Math.Log((1.0 + Math.Sin(a))/(1.0 - Math.Sin(a)));
}