Originally posted by: Budweineken
You are speculating that the bankai is when the soul slayer is summoned in to their world, when we still don't know the real nature of what the bankai actually is...
See episode 45. Yoroichi makes it rather clear that Bankai is "manifestation" of the Soul Slayer, the sword actually becoming the spirit behind it. This is almost exactly opposite of Shikai which has the spirit infuse its energy into the Soul Slayer, empowering the sword and causing the sword's shape to change.

And the animal manifestations, like monkey/snake Zabimaru and Zangetsu the person are the "materialized release", or Bankai.

Still, I have a feeling once Ichigo really gets his bankai skill down and uses it in battle, it will NOT look like Zangetsu, but much more like a Hollow.