Recently, as outlined in this CNET article, several research teams have published papers in which they explain that intelligent worms can avoid being detected by the large worm-detection and early warning networks. Those networks are run by various security organizations, such as SANS, but also by some commercial security companies.
The details are explained in that article. In essence, those networks use honeypots and the monitoring of activity on unused IP addresses to detect worm activity or capture worm samples. Both approaches can be detected with different techniques.
So, as a consequence, it should be possible to write a worm, which escapes detection of those networks long enough, to make sure that no signatures for that worm can be produced and published before it has already reached critical mass. This is yet another example of why signature-based systems are leaving an organization vulnerable for too long, in the face of potential zero day threats.
Worse yet: A fast spreading worm, such as SQL/Slammer, for example, does not even need to bother with avoiding the worm detection networks, since it will have reached all possible targets faster than any signature can be published anyway.
This just goes to show that for proper worm protection, one cannot rely on third parties to publish signatures in time. If you want to keep your network safe, you need to deploy your own worm protection, right into the middle of it. An intelligent system, such as Esphion's netDeFlect, will be able to detect a worm outbreak immediately, and provide signatures and filter instructions, even for zero day worms, that can be used to stop the worm before it gains momentum in your network.