It's worth noting that of the 28 senators who opposed the FISA legislation that included immunity for telecom companies were Hillary Clinton and Joe Biden. Voting with Obama for passage was Evan Bayh.

Arguments can be made both ways on how voting "yea" or "nay" for telecom immunity might help one's VP chances. But it would be a stark difference on the ticket should Obama choose Biden or Clinton. A difference Republicans would have no trouble exploiting.

At the same time, both Biden and Clinton have more national-security experience than Obama. Either could use that experience as a buffer against criticism.

What is undeniable is how the liberal "netroots" feel about the bill. Could Hillary's vote be a way of assuaging their previous animosity and reduce some of the opposition to her VP candidacy?