I suspect we are seeing this flurry of announcements because of the upcoming Prop. 8/DOMA case at the Supreme Court. If centrist Democrats in swing states like Warner and McCaskill are on board, marriage equality really has become the default position within the Democratic Party and a litmus test for our presidential candidates.
44 U.S. Senators now back same-sex marriage. We're getting closer to a majority.
http://en.wikipedia.org/wiki/List_of_supporters_of_same-sex_marriage_in_the_United_States#U.S._Senators
