Microsoft's E3 demo for Project Natal include two game demos -- Paint Party and Ricochet. We assumed that these minigames were developed from scratch, specially for the E3 demonstration, but Epic's Mark Rein has stated in an interview with OXM that they were made using the Unreal Engine 3.
That makes UE3 the first middleware to support the new motion control system, meaning developers who utilize it shouldn't have much difficulty adding Natal functionality to their titles. UE3 isn't quite as commonly used as it was in 2007, but this does mean there's no excuse for Gears of War 3 not to support Natal. We suspect a great many of you would enjoy a bit of motion-controlled chainsawdomizing