Okay, I see what you all are saying. It seems, however, that this is a legal argument, and a rather technical one. The states are sovereign because they are, by definition, sovereign, and the federal government only has powers at the behest of the states, because that’s what the Constitution says.
In practical terms, however, it seems to me that the states do not act as sovereign entites. Moreover, since amending the Constitution or opting out of it is beyond the power of any individual state, it seems that the individual states cannot assert sovereignty except as allowed by a majority of the federation of states [the ability to disagree with other states in amending the Constitution is another example of sovereign powers, I suppose]. It seems to me, then, that in practical terms, given the situation as it prevails in 2012 under the Constitution as currently amended and enforced, the actual execution of sovereign powers by the individual states differs greatly from the theoretical ability of a given US state to act as a sovereign state.
I guess the whole existence of the US Senate is sort of an expression of the individual states’ sovereignty, as well, as opposed to the Reps which is a bit more US-as-a-whole-by-population, though still not crossing state lines.