SpaceX have experienced what they are calling “an anomaly” during tonight’s liftoff of their Falcon I system. Early tweets on the topic indicate that there was an issue during tank separation which caused the rocket to explode. This has not been confirmed, but given the sudden loss of video from the launch vehicle, and the even more sudden termination of their webcast, it is not out of the realm of possibility.
Hopefully they have good data to help prevent this in future launches.