Please note that on Silverlight, and starting in .NET Framework 4.0, this method uses the casing rules from the invariant culture (CultureInfo.InvariantCulture) instead of the current culture (CultureInfo.CurrentCulture). This will result in this method having a different behavior on these platforms.
Instead, it is advised that the String.ToLower(CultureInfo) overload be used on all platforms to minimize the impact to your existing applications.
For more information, see: Breaking changes to the String class.