Well I guess you have to be rather particular about it, but like people are saying, it makes more sense for String.Format to be static because of the implied semantics. Consider:
"Hello {0}".Format("World"); // this makes it sound like Format *modifies*
// the string, which is not possible as
// strings are immutable.
string[] parts = "Hello World".Split(' '); // this however sounds right,
// because it implies that you
// split an existing string into
// two *new* strings.
