Originally posted by: dullard
Small microscopic changes occur between a CPU and the heatsink during the first few hours of use. For example if you use Artic Silver (or similar product) to connect the CPU to the heatsink, the hot temperatures allow the Artic Silver to flow slightly (it expands at a higher temperature) and provide a better connection for heat transfer. Thus over time the CPUs temperature will decrease slightly - until all these effects have had a time to occur - and then the CPU temperature will stop dropping.
If you are using the CPU at the stock conditition, burn-in is meaningless. The slight CPU temperature change will have no effect on you. If you overclock then this is significant. Often you will get a maximum overclock with a new setup, but after several days of use, you will be able to overclock it a bit higher. And this has been proven over and over again - but the effect is quite minor so many people say it doesn't exist.