I would also say the sensors aren't working. When I did my rear brakes this summer, I ended up breaking a few things and doing a little pounding (because one of the wheels was rusted on). When I put it back together, the ABS light was on. I didn't do anything about it because I figured that it didn't affect the regular braking system. I drove in rain and hit the brakes pretty hard to see if I had ABS, sure didn't.
I just took the dash apart and took the bulb out from behind the ABS letters.

With 160,000 on the clock, I wasn't going to spend the hundreds of dollars to fix it.