The following information was obtained from PSO World:
Sonic Team finally released information clarifying why the servers were down.
Upon investigation into the matter, Sonic Team has concluded that it was indeed the internet worm that affected networks worldwide that caused the massive server down time. Investigating the problem while the servers were up would have been too difficult. Therefore, because of all the confusion, Sonic Team had to take down all the servers to investigate and fix the problem. The servers have since then been restored and have been performing normally since the restoration.
They also apologize profusely for the problem and are sorry for all the problems caused.