With a custom operating system and the right package installed, it's not hard to get a Raspberry Pi running at 1GHz, up from its stock speed of 700MHz. But going beyond that? It is possible, but the gear required cancels out two of the main benefits of the device -- its portability and silent running.
As Joost Verhelst and Koen Crijns over at Hardware.Info discovered, it is possible to get the tiny computer running at over twice its original clock speed, however, you'll need to pick up an EVGA EPower board to feed the Pi extra voltage (1.8V, up from the normal maximum of 1.4V -- and the stock 1.25V) and a phase change cooler to keep temperatures down (to -25°C, to be exact).
At 1.5GHz, the Pi is a bit faster than an Athlon XP-M 1800+, but not as quick as Intel's slowest Atom. So, yes, in the grand scheme of things it's not worth the trouble. Nice experiment though.
Hardware.Info Pro OC overclocks Raspberry Pi to 1.5 GHz [Hardware.Info]