http://www.raspberrypi.org/introducing-turbo-mode-up-to-50-more-performance-for-free/

We’ve been doing a lot of work to understand the impact of voltage and temperature on lifetime, and are now able to offer a “turbo mode”, which dynamically enables overclock and overvolt under the control of a cpufreq driver, without affecting your warranty. We are happy that the combination of only applying turbo when busy, and limiting turbo when the BCM2835’s internal temperature reaches 85°C, means there will be no measurable reduction in the lifetime of your Raspberry Pi.

so I wouldn’t worry :)