Why not check the source, MSDN :)
Ninja edit: I just saw the link you posted at the top of your question. Select Silverlight from the other versions dropdown and you'll see the note below.
Notes to Callers
Starting in Silverlight 4, the behavior of the String.IndexOf(String) method has changed. In Silverlight 4, it performs a case-sensitive and culture-sensitive comparison using the current culture to find the first occurrence of value. This conforms to the behavior of the String.IndexOf(String) method in the full .NET Framework. In Silverlight 2 and Silverlight 3, String.IndexOf(String) performs an ordinal comparison. If the common language runtime determines that a Silverlight-based application was compiled using either Silverlight 2 or Silverlight 3, it performs an ordinal comparison; otherwise, it performs a culture-sensitive comparison.