As we predicted a couple of days ago Apple's servers are unable to deal with the excessive amount of traffic caused by the availability of the new iPhone OS 3.0.
This is a sad repeat of the past 2 major releases and does show that Apple has not learned from their past mistakes. If this is not cleared up in the next couple of days new iPhone 3G S owners will find themselves not able to activate their shiny new phones.
On the other side this could be an intentional issue. It brings press to the launch, makes people wait and want it more. In general it drives interest.
UPDATE - Good news! It seems like Apple has fed its servers some more muscle and now the activation server is back online. I'm updating my iPhone now and it seems to be working fine - CW.