By default, the Pyra's OMAP 5 runs at a nominal 1.5Ghz. One can overclock or underclock the Pyra, or really any GNU/Linux machine, using the cpufrequtils package. In Debian, type into a terminal:
sudo apt-get install cpufrequtils. Search online for cpufrequtils to see documentation and usage.
It is important to remember that overclocking any computer risks making it unstable, potentially leading to crashes and loss of data. Overvolting to improve stability at high clock rates risks overheating and permanently damaging the processor. You have been warned!
Some small discussion on the boards regarding underclocking (for improving battery life): forum. At its lowest frequency, the Pyra CPU still will beat out a GHz Pandora model. Note that once automatic power-saving support has been added to the OS, manually under-clocking is unlikely to result in any battery-life improvements.