AMD has issued the latest version of their Thermal, Mechanical, and Chassis Cooling Design Guide which is the official guideline for hardware vendors to build their products on. According to the January 2002 release of the technical white paper AMD recommends that the four mounting holes surrounding the Socket A are removed from the so called Motherboard Keepout Area. Read more...

This sums up to the following: Maximum mass of heatsink: 300g Only clip attachment will be possible The Socket A allows heatsink attachment by 3 noses on two sides of the socket using a metal clamp design to firmly seat the heatsink. Due to the maximum clip force of 24 lbs the maximum heatsink weight is limited to 300g.
Almost all high ranked heatsinks from Alpha, Swiftech or Thermaltake exceed the 300g limitation and were usually attached by being bolted on the motherboard itself using the 4 mounting holes. MSI and Asus are said to keep the holes in future motherboards while Abit's european marketing manager Thore Welling said that after their KT333 series the holes will be left out. AMD spokesperson Jan Gütter confirmed:"Scarcely anybody ever used these holes, so we decided to remove them from the specification." Really? Well I for one use them... View: AMD Thermal, Mechanical, and Chassis Cooling Design Guide | January 2002 Revision