Ok, this is the result of last discussion in python-dev.

I changed _convert_other() to make it return
NotImplemented, and check in every place where it's
called to pass back the NotImplemented (something
similar suggested Michael Hudson in the discussion: he
passed back "other", but I think is cleaner just to
pass the signal, and let the architecture to do
whatever it should).

The tests runs ok, seeing a +1.8% in test_decimal time.

I'm sending this to you, because I...

- Don't get to understand from the discussion if this
is something that should be fixed at this level or at
another.

- Don't really know if this solution is as clean as it
seems.

Thanks! 

.    Facundo