I had the problem, turned out one of the wheel speed sensors had failed. In my case the traction control light also came on, since it uses the same sensor. It was repaired under warranty, so I have no idea what the part or labor cost would be. Hope that helps.
My Daughters Focus just did the same thing. Her light is on. She called me and I asked her if the abs brakes were working and she said it didnt feel like it. What should we check and can she drive it a while with the light on?
I fixed once and issue like this, and the problem was not that the speed sensors in the wheels were damaged. They were so dirty that couldn't read the speed, so I only had to clean the layer of dirt around them and everything started working perfectly again.
If it is ABS related, you should get an ABS reader that tells you what's going on, but those are really expensive. You can replace a sensor easily with a manual and some skills. But this depends on where they are located and if it's a sensor problem and not other thing.