From CNN/ORC poll (PDF): do you think marriage between same sex couples should be legal? I've graphed the data for you. This continues the recent trend showing a steady majority in favor of marriage rights, overall, even though there are pockets of implacable opposition.
(Note, I graphed this from the first data point they included in the poll, by month. They only gave data back to June '08)
Here's a question that gets at the reason for a change. Do you have a family member or close friend who is gay? This illustrates the effect not only of media exposure, but of us COMING OUT to friends, family, and co-workers. It's much harder to demonize someone whom you know and love.