Yeah and the reason ichigo cant seal his sword, is because his spirit power is to high. I dont think it has anything to do with the hallow just his own spirit power is to high and so its in perma shikai.
Life and Death are always one and the sameEVEN IF A RAINBOW IS BROKEN THERE'S STILL THE SKY
I think (personally) that the change in Zangetsu (from his original appearance to a hollow version of Ichigo) shows that the balance of power was askew, perhaps due to the belief that he wasn't strong enough.
I also think he was strong enough, but his doubt in himself is actually what caused the change.
It is a classic good/evil, light/dark battle with a really cool twist.