It could well be a wheel speed sensor. Just which one it is, and whether the fault is electrical or mechanical, can be determined through electronic diagnostics. An independent shop might run it as a courtesy, I imagine the dealer would almost certainly charge you.
To save you that money, but would involve some labor on your part, I've heard that what you've experienced could be due to no more than a connection which needs to be cleaned. Jack up the respective corner of the car, disconnect the sensor cable, clean it out (compressed air if available), reconnect, and see if that turns the trick. If not, then it's back to step 1.
