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I think it does, but not in a way you can control.  One of my renders left a file on disk with a file extension that seemed to relate to irradiance caching but now it's gone (written and removed automatically and silently).  I don't know if this is useful to you.

It might be something Poser is using at the time of render but not later.
True disk caching, such as in Mental Ray and Lightwave, creates a GI light map or a GI samples map and stores it to disk if you have it set to do so. So if you re-render the scene, the GI is already baked into it and it doesn't have to recalculate it. That's useful if nothing in your scene is moving, such as a walkthrough in arch-viz, where only the camera moves.
If Poser 8 were doing that you would know it, as the render time would only be long for the first render and all subsequent renders would be significantly quicker.