Today, Maryland became the second state to pass the National Popular Vote Interstate Compact in both houses of the legislature. (California had done so previously.) Unlike in California, however, Gov. O'Malley has said that he will sign the bill, so Maryland is the first state in which it will become law. Of course, Maryland's electors will not be bound to the national popular vote until a majority of the electoral votes nationwide are, so it is highly unlikely to impact the 2008 election.
Arkansas and Hawai'i have passed the bill in one house with it pending in the other. Govs. Beebe and Lingle support the Compact. Colorado had passed it in the Senate, but it failed in the House; Gov. Ritter supports the Compact. Gov. Schwarzenegger vetoed the Compact in California after it passed both houses of the legislature, but has said that he may reconsider his position in the future.
The Compact has been introduced or is currently being drafted in every state/district but Nebraska, Idaho, Michigan and DC.
Such a movement will never gain enough energy to change the constitution.