[janzer]

At least in Michigan yellow lights certainly have legal rules enforcing them, from MCL 257.612 (b):

    If the signal exhibits a steady yellow indication, vehicular traffic facing the signal shall stop before entering the nearest crosswalk at the intersection or at a limit line when marked, but if the stop cannot be made in safety, a vehicle may be driven cautiously through the intersection.

And I know of at least one case of someone getting a ticket, challenging it in court and losing after failing to stop for the yellow light.

Edit: Found the relevant Illinois law 625 ILCS 5 Sec. 11-306 (a)1.:

   Vehicular traffic facing a steady circular yellow or yellow arrow signal is thereby warned that the related green movement is being terminated or that a red indication will be exhibited immediately thereafter.

So still legally defined but it seems in Illinois the yellow is simply a warning signal and does not include a requirement to stop.