99 F150 ABS Light On
I had the same problem on my F150, but it turned out to be that when I started the truck I rotated the key a little too much and the ignition on the steering column got jammed too far up (weird). Now every time I start my truck I make sure my ignition is in the right place.
This solved my problem with the ABS light staying on all the time.
Hope this helps man.
Chris