It makes for an interesting read. About a year ago the FAA converted from an old Unix based system to a Microsoft Windows based system. It appears that issues in the software require a 30-day preventative maintenance cycle. Technicians put in an automatic shutdown that would kick in if the system were left running for more than 49 days without the required maintenance. It is reported that the FAA had known about the issue for over a year, and this time around, the maintenance was missed and the system shut down.
"Risks Digest" is a moderated discussion of technology and computing risks, run by the ACM - Association for Computing Machinery. Update 9/26/2004: The Sept 25th Risks Digest has a much more detailed description of how ATC Communications systems work. Written by a controller in Seattle Center, it's a nice insight into the ATC systems.