Well, in principle ABS shouldn't necessarily result in longer stopping distance: sliding friction is lower than static friction, so if you can control the braking precisely enough, not locking the wheels should result in shorter stopping distance along with not losing control. I'm not saying current systems are that good, though.
On dry surfaces, a professional driver is able to do Threshold Braking, which in non-ABS cars maximizes stopping power by taking braking just up to the limit before the tires start sliding. On cars with ABS, the intent is to minimize the rate at which the ABS pump activates in order to maximize stopping power (pulse ... pulse ... pulse vs. pulse.pulse.pulse)
ABS has an advantage over regular systems in that the computer is able to control braking effort on a per-wheel basis. Something a human with only one pedal to push can't do. IIRC, the best ABS systems are able to cycle at about 10Hz.
Something I vaguely recall reading about is that in a panic stop on ice, the heat from the sliding friction will melt a very small bit of ice under the contact patch, turning it into a hydroplaning situation. I need to see if I can find that reference again...
"ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces for many drivers; however, on loose surfaces like gravel or snow-covered pavement, ABS can significantly increase braking distance, although still improving vehicle control." (with citations) http://en.wikipedia.org/wiki/Anti-lock_braking_system
