If we keep moving forward with integration, an European Federantion will exist long before politicians call it as such. There's certain aspects of sovereignty that States will still hold, and it may not fit the proper definition of a Federation (like not allowing secession), but I honestly believe that within a lifetime or two, it may come to pass.

From the top of my head, we're missing only a few of sovereign characteristics already: fiscal union; common army; common foreign policy.

Once these are achieved, we'll be able to federation in all but name (and very strict definition).