It seems that the latest firmware update (version 4.45) for the PlayStation 3 went live yesterday and is causing a number of people problems. Owners of the device are reporting on Sony's official support forums that after the update the units will not longer boot.
Sony has since responded and stated;
"We are aware of reports that the recent PlayStation 3 system software update (version 4.45) has caused the XMB to not display on a small number of PS3 systems. We have temporarily taken 4.45 offline and are investigating the cause of the problem. We will announce when the system update is available for download as soon as possible. We apologize for the inconvenience."
Ironically one of the improvements that the 4.45 was supposed to include was increased system stability.