I've been thinking about service contracts a little more in light of recent discussion over what (in the brave new indigo world) will define a service contract. It occurred to me that a contract based on the CLR type system will never be isomorphic with one defined in terms of an XML schema because the two type systems are not equivalent.
See: http://weblogs.asp.net/dareobasanjo/archive/2004/02/20/77264.aspx