Surely..
...An STB that can 'translate' any type of 3D TV signal for display on a non '3D TV' would be more useful?
If consumers want 3D TV, and Sky are likely to be the first to market with a channel then you can faily safely bet that which ever method they choose is likely to become the standard.. this would give a box that can display any 3D signal 'only' on any 3D TV a very limited market of wealthy early adopters that chose the wrong one.
And if they can afford to be an early adopter of 3D TV are they going to buy an STB to fix it or just buy a new 3D TV that works..