Like others, my belief is May/June. I know that puts the release in the dreaded "summer" season, but anything earlier is too early and anything later will be bad for subscriber bleed.
There's NO WAY this is going to beta before the first of the year. We're only two-and-a-half weeks from Thanksgiving in the U.S., then it's Christmas and New Year's rush. There's going to be a lot of employee (and player) downtime for the holidays and it would be absolutely foolish to start a beta when people aren't going to be able to put their best efforts into testing it (players) or tweaking it (developers).
By April we will have spent the same amount of time in SoO as we did in ToT. If Beta goes live in January/Feburary, figure 3-4 months of testing and tweaking, then a 1 month polishing period where we get the pre-expansion patch and the pre-expansion event, then live. Anyway, that's my theory and I'm sticking to it.