Nate Silver, who did a stellar job tracking the 2008 presidential election, has built a model for predicting when each state would vote against banning gay marriage. His model results in a list predicting the year each state could tip to the marriage equality side of the scoreboard. Kentucky, South Carolina, Oklahoma, Tennessee, Arkansas, Alabama and Mississippi would be the last hold-outs, taking until the 2020's.
The model is a simple one, so there are huge possibilities for reality to go differently, but it's making good predictions already, accurately guessing how California voted on Prop 8. The comments on the post are the usual debate about marriage equality, mixed in with some reports from the field ("Idaho will never vote against a ban," vs "Idaho is closer than you think," and "You've overlooked the Mormon influence in Utah," etc).
BTW, The Map Scroll mapped the data for those who want a visual reminder of where the Deep South is...
I've long thought that acceptance of marriage equality was only a matter of time, it's interesting to see Nate put some quantitative analysis behind it. In the past, I've causually tossed off 50 years as a possibility, but Nate is giving us hope that it may only take 20.