Originally posted by: Everon
Well you never know...Kabuto might have been bitten in the ass for all we know. It would raise questions though, why Kabuto didn't use the curse seal against Tsunade instead of eatting a soldier pill.
So to make a long story short. It seems there are too many holes for Kabuto to have a curse seal.
Edit:
Why Oro never bit him is another story, Here are some of my speculations
A. He's a spy. Having a viewable tattoo isn't the best way to blend in.
B. He's either too ________(weak/strong/smart) to use a curse seal.
C. Oro doesn't want him as a host. All those sound nins with curse seals were selected as possible hosts and backup bodies.
D. Kabuto's vast knowledge and abilities are more valuable to Oro rather than using his as a replacement to his body. Consider Kabuto as Oro's right hand man/ or even as far as his partner in crime.