So they wait until now to announce the arrest. Timing is everything.
If I am engaged in a war with a certain outfit, and I come into some information that I eventually have to reveal, I am certainly going to chose to reveal it at the most advantageous time for my side.
If holding to close to the vest for a month allows us to monitor those who were attempting to do business with this guy a bit longer until they realized he was not coming back, then great. If talking about it now somehow causes difficulty to Al Qaeda's current plans, then saving it for now is great by me. And if releasing the information now makes it more difficult for Daschle and crew to undermine the administration for their desired goal of taking power (at our expense), then I am for that as well.