The investigation is underway, hopefully Elon and his team will get to the root cause as soon as possible and launch again sometime later this year.
Preliminary SpaceX Internal Analysis: Falcon I Flight...: "However, at T+25s, a fuel leak of currently unknown origin caused a fire around the top of the main engine that cut into the first stage helium pneumatic system. On high resolution imagery, the fire is clearly visible within seconds after liftoff. Once the pneumatic pressure decayed below a critical value, the spring return safety function of the pre-valves forced them closed, shutting down the main engine at T+29s."
(Via Commercial Space Watch.)