The web monster from Nokia, which has an ARM Cortex A8 CPU with a stock clock speed of 600MHz, has successfully been overclocked to boot at over 1GHz. I too have tested the device at 900MHz and 800MHz speeds and I have had no stability issues at all. Especially at 900MHz, everything feels snappier, and even an already great and responsive phone line N900 benefits from the speed increase dramatically.
Now I would never encourage anyone to go out and overclock their devices, especially when very little, if not nothing, is known about how efficiently the device is cooled (passively of course) and how overclocking the ARM Cortex A8 CPU would affect its and the surrounding components’ lifespan. However, if you are willing to take the risks, you can find how by following the below links…
Here’s the loooong thread, where it all started: http://talk.maemo.org/showthread.php?t=39753
And here’s how: http://talk.maemo.org/showpost.php?p=595582&postcount=774
