CNN has a new poll that shows a slight majority of Americans support same-sex marriage, making it the third of recent marriage polls to show that:
The wording of the question varies with each (the AP only asked about the federal government and didn't say "gay and lesbian," the WaPo/ABC poll asked about "illegal or illegal" while CNN asked about same-sex marriages being "valid") but the results are similar.
All of them were polls of Americans generally, so if there were an actual vote on this issue the numbers would probably be skewed a little against same-sex marriage since older people, according to the CNN poll, as still more reticent about the subject. The CNN numbers also broke out by race (white vs. non-white) and there wasn't much difference.
Nate Silver put together a chart on this subject at the NY Times that shows that the general movement towards same-sex marriage has been speeding up in the last two years, which is probably due to a number of factors, like media showing more LGBT people on TV, politicians not demonizing us as much, and several states legalizing same-sex marriage without consequence.
But, as his chart shows, that doesn't mean that this can't be turned around. Around 2004 there's a trend in the polling, even more pronounced in the Washington Post/ABC poll, where opposition to same-sex marriage actually increased while support decreased. It's most likely because of the homophobic campaign season that year, which means that some people are still very much into group-think and are willing to change their opinion on the subject in both directions.
Although with politicians focused on deficit scare more than homo-scare going into 2012, that's probably not going to be an issue for a while.