Seems there haven't been quite as many activations as expected. 26,000 in the first two weeks as opposed to 100,000 in the first weekend..
http://www.theregister.co.uk/2007/11...e_activations/
That's not to say the iPhone has failed. It's possible a lot of iPhones are christmas (or birthday) presents, so wouldn't have been activated yet. Similarly, those who bought an iPhone and unlocked it won't necessarily need to activate it either.