I've done a ton of cord blood research, because we're having it done. I spoke with 4 doctors about it, personally, and spent hours in UPenn's library researching the medical journals both for and against clamping. The OB's conclusion was that the baby is born with all of the blood that it needs, but the cord blood is loaded with stem cells, which can essentially act as whatever the baby needs it to (i.e. speed up healing of a displaced shoulder from delivery, etc)
It was a really tough decision for us, but we have a really terrible history of leukemias, lymphomas, ALS and heart disease in our family, and the cord blood can treat, if not cure all of those. So, it just came down to our feelings that the baby would most likely be better off with the cord blood...but compared to battling leukemia, recovering without the extra cells would be a cakewalk for a healthy baby.
I'll get off my soapbox now lol