Dittos. Zarqawi is a major major capture and we would have a multitude of reasons to keep it hushed up until we can secure him at a safe, secure location for interrogation.
What also makes us delay news on a Zarqawi capture: We may also be gearing up all kinds of security for prominent Iraqis so they can't be taken hostage or murdered to secure Zarqawi's release. Other security measures too. The terrorists will want this uber-vermin back and may resort to very spectacular terrorism to compel his release.
In this case...we may never know, other than we don't hear anything from him again. I do know the rumor is widespread now, and if it is not true...we will know. If it is true, we may not ever have confirmation.