Running cooler than my single 5870, so that's definitely a plus.
Good PSUs have short circuit protection, so if a trace were to arc over to another and start smoldering, either on the GPUs or the Mobo, the PSU should kick off and will continue to until the short is gone. What make/model are the PSUs you're using?
The worry isn't the PSU continuing to fuel a fire. It just takes a spark to possibly start a fire, especially since there appears to be wooden planks and beams, the dry heat from the rig would ensure the environment is perfect to encourage a fire. Pretty much like how hot dry summers make it easy for wild fires to start. And sawdust (or dust of any flammable material) are extremely combustible.