Things are looking that way. A lot of scan buzz and such is floating around. The name Shadow has been decoded from Famitsu allegedly, and it is deduced he can throw a Sonic Boom (though it has been pointed out that Seth, one of a number of clones, can also).
I'm pretty damn sure he'll be in. Gouken was created for SFIV to the elation of SF fans. Charlie/Nash/Shadow is the next big ticket item on the fan wishlist and he fits the thought-dead master mold, so I think he's a lock.