ARM based processors can be overclocked and we've seen Tegra 2 overclocked from original 1GHz to 1.5GHz and in some cases people have managed to overclock native 1GHz CPUs to even more than 1.5GHz. You do need to root your phone, get a super user access, which is a sort of hacking, and you need a special kernel that supports overclocking, and there is hardly a better place to find all the necessary information that XDA developers forum, where many hard working people find a way to override built in safety nets.
These guys have found a way to overclock Motorola Xoom and they did a good job in overclocking LG Optimus 2X from default 1.0GHz to 1.5GHz. Many will like the fact that overclocking your phone brings an option of under-voltaging where you can set lower voltage and run your CPU underclock or at the same stock level, and still save a bit of your battery life.
Overclocking phones definitely is becoming a big thing and more and more enthusiasts are toying with it. After all it’s all about the highest score. Some instructions of how to do it with LG’s Optimus 2X are here.