It isn't, but if he does play how far will he get given that (a) he's going to be very rusty and (b) this isn't one of his favourite surfaces? I'll be very surprised if he plays but I can also understand why things have been left hanging in the air.
Valid point Aileen, but maybe he wants soem game time other than just an exhibition in Dubai. We know how competitive Nadal is, and how much it meant to him after working so hard to get to World Number 1. Maybe he's wary of the fact Andy will probably be more of a threat on Clay next year due to Lendl's training advise, and he'll want to stay well in touch with the others in regards to ranking points.