Indeed, monetary types solve different problems than decimal types. Financial applications do have a need for these kind of implicit error checks.
But this is easily done by creating a custom class -- which has the advantage that the set of constraints can be specialized to the needs of a specific application. When we add a monetary type to the language we'll never get it right for all apps. OTOH, I think we could get a fixed point type right.
How many other languages have a monetary type?
What support for money does SQL have?
--Guido van Rossum (home page: http://www.python.org/~guido/)