Some good answers I think. To give a practical example that explains the previous posts, suppose you are flying along in straight and level at 100 kts and 2000 ft, with wings level and 0 degrees of pitch attitude, and encounter a sudden nose up disturbance (wake turbulence, for example.) The aircraft response will be as follows:
a) No SAS or ATT system - the aircraft will pitch nose up, and will continue to do so until a pilot input is made.
b) SAS system fitted - the SAS will sense the rate of attitude change and attempt to counter it, so eventually (quite quickly) the nose will not be rising or dropping. However, the will be a new pitch attitude, that will be higher than previously, and will lead to a deceleration and climb until the pilot adjusts the attitude himself.
c) ATT fitted - ATT systems have SAS as an integral part of their function, so the SAS will counter the pitch up, as described above, and then the ATT system will further adjust the aircraft attitude to wings level and zero degrees pitch. This pitch attitude will result in the same IAS - more or less - but the aircraft will have climbed and will now fly level at some height above the original altitude.
d) ATT with Upper modes - if you have a coupled autopilot with Altitude and IAS hold, you will get a combination of rate sensing, attitude sensing and external barometric data, all combining to give continued flight at the original datums.
Hope that makes sense!