If there weren't already enough reason to encourage payment for blood donation (counterarguments to which are ably rebutted by Alex Tabarrok), we might have another.
Bone marrow donors, who donate through peripheral blood stem cell transplantation, can now be compensated in the US under the same provisions that let blood donors be compensated. PBSCT seems more intrusive than blood donation, but somewhat less intrusive than standard marrow donation [see here for one discussion].
Demand for marrow seems likely to increase: bone marrow transplant may be effective as treatment for HIV. If these kinds of results hold up, expect some very effective political campaigners to start helping to change legislation to encourage marrow donation in places where compensation isn't yet legal.
I donated blood a half dozen times as an undergraduate; as I've yet to make any withdrawals from the system, I'm refraining from any further donations on principle until policies around compensation are changed.