24:00:00 allowed or not?

ISO 8601 section 5.3.2 allows 24:00:00 as a representation of midnight. 
Appendix D of XML Schema Part 2 says that the hh field runs between 0 and 
23, but there is no mention of 24:00:00's not being allowed as a difference 
between XML Schema and ISO 8601.  I'm left uncertain as to whether 24:00:00 
is allowed or not.

James