According to Scott Beale at Laughing Squid:
Wow, this is a shocker. At around 3 am Tuesday morning at Burning Man 2007, during a rare lunar eclipse, the Burning Man sculpture was set on fire prematurely (it is normally burned on Saturday). The Black Rock City Emergency Services Department was able to put out the fire in time and salvage the sculpture (it had not yet been loaded with fuel or explosives). It is still scheduled for its normal burn on Saturday and they will be working throughout the week to repair any burn damage and re-install the neon.
Lots more info, pictures and updates at Laughing Squid
UPDATE: arsonist arrested
Link to SF Bay Guardian blog