> This behavior seems to be required by the General Decimal Arithmetic
Specification (http://speleotrove.com/decimal/daexcep.html )

Yes, exactly.  The decimal module strictly follows that specification.  I don't like the 0**0 -> NaN result much either (especially when we also have inf**0 -> 1), but it's what's specified.  I've talked to Mike Cowlishaw (the author of the specification) about this particular issue, and the spec is not likely to change on this point.