Hamm is currently planned for sets 7596 (Trash Compactor Escape), 7597 (Western Train Chase), and 7598 (Pizza Planet Truck Rescue). These will be the first time we've seen Hamm in minifigure form.
Minifigure Parts and Accessories
Hamm appears to be a single piece with no articulation. There's a slot on his back, which makes sense since he's a piggy bank. The box pictures show coins falling out of Hamm, and as The Brick Show confirmed, he is functional with LEGO coins. Hamm also has a rubber stopper on his belly.
As we've suspected from other set pictures, it appears that Hamm may come in two core variants, one "clean" and the other "soiled". It is unknown at this time which version would be more sought after by collectors.
In the Western Train Chase (7597) Hamm appears as "Dr. Porkchop" and with that comes with a black bowler-type hat. As can be seen in the box picture from Trash Compactor Escape (7596), there is a square shaped hole at the top of his head, which is likely how the hat attaches.
Army Building Potential
I imagine the ABP for this figure is low. 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 Hamm has appeal. In his Dr. Porkchop disguise, the hat may also be of interest, but it's unclear at this time how it might attach to other LEGO pieces and minifigures.
(Pictures from Amazon.com and Eurobricks)