I think a couple William Gibson novels start out this way: S.F. officials locked out of computer network
A disgruntled city computer engineer has virtually commandeered San Francisco’s new multimillion-dollar computer network, altering it to deny access to top administrators even as he sits in jail on $5 million bail, authorities said Monday.
This is likely the result of a failed management process rather than a technical vulnerability. In fact, the system must be pretty secure if they are still trying to get in!
With systems this large, you have to have audits and monitoring of everyone with significant access, including those doing the monitoring. Otherwise, stuff like this becomes possible.
Plus, this guy was a known problem for quite some time, apparently. Always make sure you’ve secured the network before trying to terminate the top system administrator.
(Via Deane at Gadgetopia.)
C. David Gammel, CAE 
Wasn’t the Gibson book you’re thinking of Idoru? Although you could make a case for the Neuromancer series (admittedly, there it’s the computer taking over itself rather than a person doing it …)
(Yes, my previous comment was extremely geeky. I embrace my geekiness!)
I was thinking of computerized hijinks starting in San Francisco in general. More of a consistent theme than than an exact parallel.
Be proud of reading Gibson! He’s got a bead on the near future like no other. 🙂