WKMG in Orlando reports that an unmanned resupply ship commissioned by NASA for the International Space Station has exploded seconds after launch. The craft was an Antares Rocket and Cygnus Resupply craft combination supplied to NASA by contractor Orbital Sciences Corporation.
Update: Video of the explosion
Interestingly, the engine used in the rocket was a modified soviet engine that apparently has a history of failure: https://en.wikipedia.org/wiki/NK-33#History.
I remember a bit of discussion about this when the launch was delayed on Monday. Apparently repackaging Cold War surplus counts as "commercial" spacecraft.