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Our revision of Part 2 has arrived at a reasonably clear distinction between these two concepts. Part 1 did not, historically, make much distinction between the two. We should survey all uses of either phrase in Part 1, and make sure their usage agrees with the new usage in Part 2.
"simple type (definition)" worries me slightly. We certainly have datatypes, and Simple Type Definitions, and they are certainly different. Each Simple type Definition selects a datatype and associates with it at least a name and a position in the "construction" hierarchy. I suppose a "simple type" might be construed as, perhaps, the triple of name, hierarchy position, and datatype, but we have not defined the term 'simple type' and have been careful (I hope) not to use it in Part 2.
I have now reviewed all of the instances of the term 'datatype' in the Structures spec, and as many occurrences of the string 'simple type' as patience would allow; I have found no instances of either which seem to me to require change. There were numerous places where Structures seemed to me to be playing a little fast and loose with the intensional/extensional distinction which I understand to be the point of having the two terms, but in all those cases Structures proved to be following the usage of the Datatypes spec. So there may be a weakness in our usage of the distinction (if there is a reliable distinction to be made), but no inconsistency between the specs that I have detected. Accordingly I propose that we close this issue as FIXED. Others with a finer nose for semantic inconsistencies are welcome to examine the spec and propose changes to the text if they wish.
The WG reported this bug as FIXED on 2009-09-04. We are closing this bug as requiring no futher work. If there are issues remaining, you can reopen this bug and enter a comment to indicate the problem. Thanks very much for the feedback.