Personally, I'd like to see a united EU army. I don't see any problems regarding sovereignty -- as was said before, NATO members cannot make a fully independent, sovereign defense policy anyway. We depend on America, which is mostly a good thing, but it also means none of us in Europe is truly sovereign.
And think about the EU what you will, that it's bloated, lacking democracy and meddles too much into the affairs of the member states -- all right, to some extent, but if there is one field where integration actually makes sense, it's defense and the military, IMHO. I'm not comfortable with some things the EU does, and I believe Europeanization of policies should be limited (I'm a federalist: Leave at the lower level as many fields of policy as possible, certainly everything that has to do with culture, and only transfer to the national or European level only what can really be better done there), but defense is one of the few things I am thoroughly convinced could more efficiently be done on European level.
Most European countries are very small and could not even properly defend themselves. Even the larger European military powers are only medium-sized in international comparison at best, and even they wouldn't be capable of sufficiently defending themselves without the NATO. Europe, small and larger countries alike, are freeriding on US military power, and as reward, we keep annoying the Americans by complaining time and again whenever they actually use their forces. That doesn't strike me as a good status quo, neither good for us, nor for the Americans on the long run.
Only a united Europe could build up a really effective army capable of defending Europe. No European country could do that alone, or even against other Europeans. We'd actually be more sovereign if we had a united European army, because we no longer depend on the Americans (which has many obvious advantages, but certainly disadvantages too). And it would be more fair towards our American friends, if we actually took some more responsibility and allowed them to spend less money for our defense.
Of course, it's a crucial question how such a united EU army should be structured, and how the problem of different interests can be solved. I'm not sure how this should look like in detail, but I think it's doable. Also, it should probably not be compulsory for all EU members, but like the Euro currency, a project started by some members, while others are free to follow later. That would allow those countries which are too uncomfortable with giving up their army, or with European integration in general, to stay out.
For example, I understand the British are very uncomfortable with giving up their army, that has a very long and shining tradition, and with pooling sovereignty in general. Also, they don't like federalism (although I guess they'd appreciate it more, if they saw it working first hand). That must be respected.
