You can already pre-order the 3D version in the Amazon UK store but Disney seems to have put a US 3D release on ice: Official: ‘Frozen’ 3D Blu-ray Domestic Release Delayed Indefinitely – Disney News by Stitch Kingdom.
There is a report that the UK release is a “region free” version (it is labeled that way at the Amazon UK site). Otherwise, as you may know, DVDs and Blu-rays are coded by region so you cannot normally purchase a disc overseas to play in U.S. players.
Why is Disney not releasing a 3D version in the U.S.? We can only guess. Sales of 3D Blu-rays skyrocketed in the past year so there seems to be consumer interest. Perhaps they plan to release the 3D version later, bundled with a “behind the scenes” video. Or something. My guess is that they will eventually release a 3D version – and maybe their marketing figured out that many will buy the 2D version now, and they can then sell the 3D version to those customers later! And make more money that way! Argghhh!