They aren't talking too much about this at CES today...at least not to most normal folk. I did catch a glimpse of one that was out on top of the new 120-300. It looked nice. A little smaller than the 1.4x canon extender I'd say.
They were expecting March for release right now, and no office costs but I heard "it shouldn't be too much" and "about the cost of an extender". I'm really hoping that's sigma prices and not canon!
The main functionality is to really be able to adjust lens parameters, like focus speed and potentially IS/OS settings if the lens has them. I didn't get a chance to ask to what extent you'll be able to customize things, but I imagine there's room for things to change in the next couple months anyway.
The new 120-300 also looks really nice. I didn't get to hold it, but I did play with the previous version and it wasn't bad! Certainly for HALF the cost of the current Canon it should be a good deal.
I've been using the new 35 1.4 and I've got to say I'm impressed with the new sigma stuff. This is my first Sigma lens I've owned as wasn't overly convinced about the earlier stuff, but so far it's great! I'm definitely going to be considering the dock when it's released, assuming the price isn't too much, and I get more than just firmware updates and AF speed.