Smoke simulation with a domain with negative start time?

In the documentation it says that a negative start frame can be used with the particle system so that the smoke appears before the first frame (which is what I need for my project).

However the smoke domain does not let me set a start frame before (negative) the animation start frame. So I can’t make use of the particle system’s negative start frame.

I’m using Blender 2.70 btw. Is there something I missed?

Btw, if I type in the start frame for the smoke domain to say -30, and then bake, all I get is a still (the smoke is not animated)