I'm not *exactly* sure how a dimmer switch works, but if there's only one length of Romex wire running from the wall switch to the light/fan, then I don't think it's possible to do what you're asking about. Not without running another wire (from a non-dimmed switch) to the fan.
I agree with the previous poster: the cheapest and easiest solution is probably just to buy a ceiling fan/light fixture combo that comes with a remote dimmer for the light. i.e., the dimmer circuitry would be inside the fixture, post-fan and pre-light.
If you're really handy with electrical work though, and don't mind going up in the attic...routing another wire to the fan could be cheaper/easier. That would likely require some surgery to your light/fan though, since it currently only takes 1 power wire.