The building [where the fire broke out], also known as Valley Green Six, is one of the most famous buildings on the Apple campus, as it is known to house a number of hardware-based research and development projects that are underway at the company.
Not good news, but now that everyone is out, safely and soundly, perhaps I can overact with: Merciful Buddha, I hope they got the iPhone 4G (and 5G?) out in time!
As to the cause, while MobileMe jokes and quips about someone trying to boot up one of the secret new quad-core MacBook Air's, turns out:
Hal Rooney, deputy chief for Santa Clara County Fire, said the fire may have started under an air conditioning unit on the second floor. Local television station KNTV reported that maintenance crews had been working on the air conditioning systems in the building earlier in the day.
Again, everyone here wishes well all the Apple employees, their families, and loved ones, and speedy recovery for the Apple R&D team and their continually universe-denting R&D team.