From Igor Stoppa, the engineer who gave the warning. Takeaway messages:Don't use temperature sensor readings as a guide to what is or is not going to damage the system The N900 can not be expected to handle sustained overclocked heat production as well as a phone using metallic casing. More generally: device heat sinking is an important parameter. The operational voltage (as was stated by Matan) is the main reason for the decreased lifetime when running at top frequencies. What's not directly included in that short message, but is known for modern CPUs, is that operating at elevated temperatures accelerates the process known as electromigration, which eventually leads to a short or other fault in the processor. It's a cumulative effect, so e.g. if the CPU is at 40C and top voltage for 1 hour, the decrease in lifetime could be equivalent to 100 hours at 30C and bottom voltage. It's a consequence of the exponential dependence on temperature of the relevant physical processes. Realistically, if you are going to throw away your N900 in a year or even two, overclocking the top speed step to 800 MHz and using it normally is probably not going to be a problem. Edit: oops Titan beat me to the link.