Benjamin wrote (snipped):

Typeable would be completely safe if the only way to declare instances would be to derive them, but this is only practical if it can be done from anywhere outside the data type definition.

Unfortunately this would also outlaw some legitimate uses of Typeable. In particular, I think you can only derive Typeable for a type constructor of type (*). GHC has recently added Typeable1,Typeable2,... which are classes of type constructors of kind *->*, *->*->* and so on, up to 6 arguments I think, and these can be derived, which is a great help. But there are still kinds this does not include; for example (*->*)->*, which is an example of a type constructor I actually used where I wanted an instance of Typeable.