Although many say soreness is not an indicator of growth, I use it a a gauge to tell me whether or not I had an intense workout. Needless to say, on this type of program I can barely move the next day. Its all about high intensity, and I think the 5 x 5 program fails to test your absolute true strength. . .it likes to stop you at a fixed rep. . .why go for 5 when you can really get 7 or 8?
Anyway, soreness is based on lactic acid buildup and microtears in the muscle.