I agree with the first answer - get it scanned with a vag-com. Everyone above is right that it MAY be the control module.
Does it ever go out?
The other issue that could cause this is the brake light switch at the pedal. I had a situation in my mom's car where the two lights would come on randomly while driving. I couldn't figure out what it was - tried going over pumps, turning, whatever - nothing triggered it on a reliable basis. It turned out to be the brake light switch, which I had previously replaced, that wasn't positioned correctly.
Basically, the first little bit of travel in the switch triggers the "test" switch for the car, and the rest actually triggers the brake light. I didn't have it set so the pedal compressed the switch all the way when it wasn't being pressed, which made the computer think the switch was being hit all the time, but the lights were not coming on - triggering a fault in the system.
In other words - it could be a simple issue or a big issue (like the module) - vag-com will be a big help.
*edit* - I thought you meant ABS & ESP - but it looks like you're talking about ABS and the brake light in the center cluster display itself..if so, disregard most of the above, but I still think a vag-com might help.
Last edited by jseklund; 03-01-2011 at 09:40 AM.