There is no even remotely firm date at this point. Late January was thrown around as a guesstimate IIRC, but it can change by a wide margin on no notice. I kind of expect it in March, just so it can conflict for my time with the Diablo 3 expansion and Age of Wonders 3.
They have said that they'll provide advance notice if the founders packages are going to no longer be for sale, so there will be advance warning of that.