Doing things that are costly isn't the only way to reliably signal. In this case the rabbit reliably communicates awareness of the fox's presence. It cannot be fake because the rabbit must look in the right direction. The fact that it's prey is not unaware of its presence is always going to be useful to a fox. It will still attempt to chase aware rabbits some times but the exchange of information will help both creatures in their decision making.
This is an equilibrium that all else being equal will be stable. Speed will still be selected for for exactly the same reason that it always was.
So 'noticing the fox' signals that the rabbit notices the fox and will run when it sees the fox beginning to chase. The fox uses the signal thus: "If the rabbit notices me it gets a headstart. With such a head start, and the fact that the rabbit runs at a certain minimum speed, I would not be able to catch it".
Even though the reliability of the signal is independent of the running, its effectiveness/usefulness depends on the rabbit's speed.
Once we have the free riding rabbits placing resources into noticing and away from running, foxes will reali...
Sweet, there's another Bloggingheads episode with Eliezer.
Bloggingheads: Robert Wright and Eliezer Yudkowsky: Science Saturday: Purposes and Futures