These have been in the news for a while. Ars technica wrote a detailed review. I found them after seeing David Merill's TED talk on his Siftables research. I bought the basic set plus three extra cubes. Here's what it looks like in practice with the six cubes, charging dock, and USB wireless link plugged into my laptop:
The SDK uses C# to build a program that runs on the laptop. The program talks over wireless to the individual cubes which report back events such as cube shake and tilt, cube touch and button push. The drawing API seems spartan--mainly drawing images onto individual cubes--so I think none of the game play happens on the cubes. Loading programs onto the cubes takes a while (15 to 30 seconds for what I've tried) so all the images are local to each cube and drawing is smooth. Once I get going with the SDK I'll know a lot more. (It's possible that the Sifteo wireless creates a lot of traffic that interferes with my laptop wireless network--shutting down Sifteo sped things up considerably.)
My three year old son played with them and there's a lot of potential. It was fun watching him learn how these work--more on that tomorrow.