Nate Silver, an analyst who has correctly predicted outcomes of many different elections, has looked at data including whether a state has previously passed a ban on gay marriage, the importance of religion to local voters based on a Gallup survey, and how many white evangelicals live in a particular state. Based on that, he created a model to predict when voters in each state would be likely to vote down a ban on gay marriage.
Here's a run down by state of when marriage equality is expected to come:
Vermont (already equal as of April 7, 2009)
Massachusetts (already equal as of May 17, 2004)
Connecticut (already equal as of October 10, 2008)
Iowa (Equal as of April 27, 2009)
Asterisks indicate states which had previously passed amendments to ban gay marriage.