there is no way to reset it on your own. correcting the problem is really the only way it will reset itself. it's not like the engine's ECU which is user-resettable, the ABS has its own separate brain. could be something as simple as a wheel sensor or fuse, or something really fuxored like the hydro pump or electronic controller.
when you did the rotors, do you mean you installed new ones? also, when the ABS light first illuminated, did it turn on as soon as you started the car? or did it turn on after you started to move (wheels started rolling)? is it intermittent? or does it stay on continuously? if it only went on after you started rolling, or if it's intermittent, it could be just a bad or dirty sensor, which is easy to clean/replace. or, if you installed a different size tire, it could also trigger a fault.
best thing to do is get it diagnosed by the dealer, they often do it for free. once they pinpoint the problem, you don't necessarily have to have the DEALER fix it, espcially if it's only a sensor or fuse or relay. if it's the hydraulic unit (aka "the pump"), it'll usually say one of the solenoid valves, or power to one of the valves. because of liability, BMW will tell you that you need a new pump (big $$$), but you can actually DIY rebuild it for free.