I don't think it's anything to really worry about. Congress has sent thirty three amendments to the states in a period of 225 years. There have been over 11,000 proposals in that same time period[1].
Theoretically possible, sure. Fascinating, absolutely. Likely? Not really.
The constant eroding of the State's sovereignty and the consolidation of power into the Federal Government's hands make this somewhat more troubling in the future however. Especially as we move towards more pure democratic systems and away from representative republicism. See for instance the National Popular Vote Compact: http://en.wikipedia.org/wiki/National_Popular_Vote_Interstat...
A similar compact for approving amendments could be an issue for instance.
I'm curious as to how making the presidential election decided by popular national vote has an effect of "eroding State's sovereignty"? As I see it, a national election decided by a national vote is the most natural thing in the world. The only effect of arcane (and frankly, absurd) electoral college system is to put disproportionate power in the hand of some of the states at the expense of other states.
The electoral college (and similarly, the now defunct election of US senators by state legislatures) only seems unnatural in the context of what states have become. They're no longer sovereign states with a central federal government that takes care of a very important but limited set of things. They're now closer to administrative districts in many respects.
edit: I mixed up my EU facts below. The European Parliament is filled by direct election. It's the Council that is elected by the executive branches of member states, and is thus more analogous to the US Senate before switching to direct election. The EU is complicated for my American brain to keep straight! I still might have some of this wrong.
Compare the electoral college to the European Parliamentary elections, which are decided not by all EU citizens but by the Council, or the President of the European Convention, who is elected by the European Parliament.
They're no longer sovereign states with a central federal government that takes care of a very important but limited set of things. They're now closer to administrative districts in many respects.
In other words, they are states -- as opposed to nations (as per your EU example).
If you read the amendment process, the states can get together and amend the constitution if pushed too hard by the Feds. It hasn't been done, but it is a valid avenue.
Theoretically possible, sure. Fascinating, absolutely. Likely? Not really.
[1]: http://en.wikipedia.org/wiki/List_of_amendments_to_the_Unite...