Rsnubje from Hardware.info has recently been trying out all sorts of tricks to get the Raspberry Pi to overclock as high as possible. He started out by mounting a single stage phase-change unit to the RPi and found a 50MHz gain. Then Rsnubje mounted an EVGA EPower to that poor little single core ARM compute "unit. The PCB of the EPower is larger than the PCB of the Raspberry Pi - it is a funny sight. Again progress was made in terms of overclocking.
Today Rsnubje combined both. He reached a frequency of 1.5 GHz - more than double the default speed - and set a new top score for HWBOT Prime in the Raspberry Pi category with 795.18 primes per second ("PPS"). That is 12 PPS higher than the previous top score. According to Rsnubje there is still a bit left: "There is a lock to go above 1500MHz :( I'm pretty sure it can go higher."
Maybe the developers of Raspberry Pi can help out our friend from the Netherlands. Until then, awesome stuff!
By the way, the performance of the Raspberry Pi in HWBOT Prime is interestingly close to that of a 1.6GHz Silverthorne Atom Z520. Note that the Raspberry Pi is running a Linux based operating system whereas the Atom is doing the same work on an unoptimized Windows 7 OS.