Just found out WCF does not supporting XmlSerializerFormat for custom fault details (see note under Creating Strongly Typed Faults section here). Everything else in my contracts is defined using this approach and now I have to use DataContracts for my fault details? What's up with that? Seems superbly lame and inconsistent to me.
Gonna have to take this one to the forums and possibly to the bug database...
Update: No response to my forum post as of yet, so I entered a bug. Go vote on it if you agree that this inconsistency in the API is "teh suck".