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:
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.
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)