The European Commission would not sign ACTA at the Japanese ceremony.
“The EU has not yet completed its internal procedures authorising the signature, therefore it will not be signing ACTA at this event,”
Closer to reality might be the description that the EU Commission cannot sign ACTA because the current version of the agreement goes beyond the present Acquis. I wonder if member states could sign ACTA or if that would be disloyal to their EU Treaty obligations. Would be quite useful to quickly invoke the ECJ to clarify the situation.