Rex is currently planned for sets 7597 (Western Train Chase) and 7598 (Pizza Planet Truck Rescue). These will be the first time we've seen Rex in minifigure form.
Minifigure Parts and Accessories
Mariann Asanuma was able to handle Rex at the recent Bricks by The Bay event. In her own words:
As I actually held this figure at Bricks by the Bay I can answer what he is like. His legs are both independent and removable. They are attached to the body by technic pin attachments in the legs. The tail is attached by a technic axle and can be removed. The arms are one part and rubbery.
There are no known accessories for Rex at this time.
Army Building Potential
I imagine the ABP for this figure is low to moderate. Unlike the army men, there appear to be no parts or accessories that would be sought after in multiples.
Big Draw
I think if a collector is seeking to fill out a LEGO Toy Story collection, then Rex has appeal. The detachable tail may have applications for builders or custom minifig hobbyists. Beyond this we will have to wait and see...
(Pictures from Amazon.com and Eurobricks)