By "Working on", do you mean actually building one to sell in retail? If so, then I would guess that the answer is NO.

However, if you mean if they are playing around with ideas, making prototypes, or doing research and development, then my guess is that they are. They probably have been for a couple years now. When will we see an SLS printer from MakerBot. I don't think it will be for 2-4 years.
Maggie