By being "absent", Kerry is not immediately tied to the accusations that Clarke has started making. When Clinton staffers testify before the 911 commission, they'll be amplifying this attack on BushCo.
Kerry's staying away from the fray, if not exactly above it. I think this is sound strategy.
If Bush loses his credibility on the "War on Terra", he's lost the only pillar holding him up. By attacking his credibility, instead of simply attacking his decisions, his critics are going at the heart of Bush's appeal -- many people think he's a regular guy that can trusted.
Currently, Bush jumps ahead in the polls if there's another attack. If his credibility is destroyed in this area, even that won't help him.
Shatter that and $500 million in negative ads won't be enough to keep BushCo afloat. Elections with incumbents are always referendums on the incumbent.
I still don't like Kerry much, but I think his campaign is playing this one pretty smart.