If an overload of a method is marked obsolete, ReSharper's IntelliSense show the method as obsolete.
For instance, the Enum.ToString method has four overloads, of which two are obsolete. Because of this, IntelliSense shows the whole ToString method as obsolete by showing it in strike through font.
This problem is a bit similar to http://devnet.jetbrains.net/thread/293407?tstart=0 (not showing method if any of its overloads have EditorBrowsable.None). In both cases IntelliSense applies an attribute that's set on a single overload to the whole method.