The EU has struggled mightily to try and get its member states to get in line behind a coherent, "single foreign policy". Obviously, we are a long way from seeing a EU where the member states do not have substantial policy differences, but I think the more moderate goal of having a singular foreign policy on MAJOR issues at least is much closer.
The proposed EU Constitution strengthens the notion that the EU desires to speak with one voice in major foreign affairs issues. The proposed EU Constitution seeks to strengthen the EU Foreign Affairs minister and provide accountability of that position to the European Parliament. Under the proposed constitution the foreign minister would be appointed by the European Council and approved by the Commission President.
Assuming the EU is successful in creating foreign policy coherence amongst its members, the question to the rest of the world (and obviously the U.S.) is when (not if) the French and British seats are merged into a single EU seat on the UN Security Council. What degree of foreign policy coherence is needed, and would one less Security Council seat be an obstacle in creating such foreign policy coherence. Obviously, the British and French would be opposed to this, but it is likely that the other member states (especially in Eastern Europe) would be interested in having an EU rep to the Security Council.