[*DIVA!]

Clinton was not present to vote on the critial amendment (Dodd Amendment S. 3907), which would have removed the amnesty provisions from the final FISA bill (S. 2248 FISA Amendments Act of 2007). Barack Obama was present and voted in favor of the Dodd Amendment.
The Dodd Amendment failed on a 67 to 31 vote and one could argue after the fact that it would not have mattered if either were present. However, before voting began on all the FISA related amendments, there was no guaranteed way of knowing how the voting would turn out.
Reasonable guessing would have likely indicated a potential loss on S. 2248, but until after several amendments failed prior to S. 2248, one could not have made an educated guess on the final outcome. If (that's a big if) the votes had been close leading up to S. 2248, Clinton's vote may have been critical. But that is moot. She had already planned to miss the voting regardless of what the potential outcome may have been.
Even John McCain participated in voting on the amendments yesterday, which was about the only time he has voted in approximately one year.
For what it's worth, I think it makes both of them look bad to the base to miss part or all of the votes on such a critical issue.