Computer?
Greg,
all the computer can do is to use the input from the knock sensor - and retard timing until knock disappears. With the spark retarded, the engine feels more sluggish and less responsive - therefore, you'll likely see worse gas mileage on lower-octane gas.
It does not contradict Thom's last statement - you _could_ get more power if there was no detonation.
It is actually a very simple feedback loop, much simpler than the other ECU tasks. You can retrofit older vehicles with a knock sensor and ignition control circuit.
The difference can be substantial - my old '68 Wag's Buick 350 would run beautifully with 5-7deg of initial advance on 89-grade gasoline, ping like crazy on regular, but if I retarded the ignition to originally called-for 0 degrees, it would feel like a 4-banger. Having a knock sensor and ignition feedback would've helped a lot.