kabuto cannot resurrect the white fang because he moved on to a better place/ his soul is at peace =/
remember that scene where kakashi is talking to his dad? just before kakashi was brought back from the dead, his dad told him that he was moving or what not?
i think he can. Impure world resurrection returns the soul back from the pure world into the impure (real) world. he has brought back lots of good guys as well as bad guys, i'm guessing a fair amount of them had moved on like sakumo did
back on topic, sakumo is not really significant to the storyline, even less than obito i'd say. also his character already did what he had to do when he met kakashi, it closed off his story nicely. but i'd still like to see what he can do, as long as his edo tensei gets a fair amount of time to fight. i was really looking forward to hanzou and it wasn't done well at all.