Someone asked Playtonic on Twitter if the ToyBox will be coming February/March. The response was that it's still too early to tell, but they're hard at work. It was originally planned for December I believe.
It's been said that the ToyBox is totally separate from the main game, so I'm a tiny bit concerned that Playtonic are putting too much time into the demo, inevitably delaying the final release by quite a few months.
But then again, Banjo-Kazooie also had the temple test level which was never used in the final game, so I guess it's a tried and tested method! /relax