I subscribe the the theory the Kabuto doesn't need a curse seal. Remember during the second exam when he was loosing to the replicating nins and his eyes started turning red? I bet he was about to unleash something very unpleasant and very powerful. I hope we get to see it at some point in the future.

I also still nurse hopes that he'll come around and leave the dark side. It's those glasses...damn, but those are kick-ass glasses, and he wears them so well.