GM Claims, Sparkplug wires and leaky valve covers cause this. Sparkplug wire loction (fixed with plastic retainers) and oil seeping from the valve covers onto manifolds (manifolds would have to be way hot to cause a fire, should smoke like crazy if there is that much oil burning).
I call BS, we've seen too many people have problems with fuel lines leaking, and fuel on the hot manifolds could easily result in the above. It is rather common.
If you plan to fix yourself, make sure you do it right. Just sayin.