Well, it is simple.
When a member of a Union (in this case the EU) shares its decisions but holds the option to exit any time it wants (which the new (Lisbon) EU Treaty provides for, then said member has full Sovereignty!
Why? Because it can dis-engage itself from all ties and return to "full independence" (for the EU) any time it wants. When one has such power, then IMO one has Sovereignty when while in the Union.