So, it seems to be a trivial matter to use a different seed on each frame (it looks nicer in animations). I.e. type "#frame" into the seed value and viola! You now have a driver. In fact, if I turn on "auto-run python scripts," it even updates interactively.
HOWEVER, this appears not to work in the node editor. I have a texture that I'd like to animate (by driving the location in the mapping node), but the driver I've added doesn't get updated; neither interactively, nor even when I render. It will update each time I open Blender, but never again.
Anything I'm missing? Or does this just not work?