I don't quite understand why you would want to lower the speed manually (unless you are bothered by the fans? But I personally don't find the fans loud at all when idle).
If you have an Intel MBP, the Core 2 Duo processors have Intel SpeedStep technology, which means whenever your CPU isn't doing intensive work (idling at desktop or a website), it is automatically down clocked, and its voltage is lowered. This is built-in to the CPU itself and you don't have to do anything at all. Of course as soon as you launch some CPU intensive application the CPU will be back to its full speed. Hope this helps.