no US constitutional amendment is needed to adopt any new voting scheme
one state could change schemes on its own
It's true that any individual state could decide to appoint its electors via whichever other system, but the Constitution still requires the electors to vote via plurality/first-past-the-post (according to the 12th Amendment, IIRC). Thus, if you support a minor candidate and he may have a chance to win your state with your state's voting system, there's still a good reason to vote only for a major-party candidate, since your state's minor-party electors would still be "wasted" in the final electoral-college vote.
Even if you abandoned it, you'd still not have proprortional representation, meaning only one party would have access to the executive power, instead of a coalition representing a congressional majority, so third parties would still have to win the election to become part of the executive. You could modify the form of the presidential administration and the congress to support proportional representation, even while keeping the electoral college, so that like most other nations, you'd have a cabinet of ministers from different parties, each representing a certain percentage of the voters, with the entire executive perfectly representing the result of the vote, instead of just representing who got the most votes.
An independent executive is a feature, not a bug. I live in Israel, which has a proportional parliament, in which the Prime Minister must have support of a majority coalition. The effect is that the Prime Minister has to bribe every little one-issue party who managed to get more Knesset seats than the margin the coalition has over the majority (thus increasing their actual influence far above what their electoral success would indicate). The US can no doubt improve its electoral system greatly, but a parliamentary-style system would not be an improvement.
Slashdot Mirror
The Latin is right, but I suppose it's not a quote from Bierce anymore.
"Cogito cogito ergo cogito sum (I think that I think, therefore I think that I am.)"
Bad Latin. You mean "Cogito me cogitare, ergo cogito me esse".