There's a whole thread on overclocking which also mentions overvolting
though I couldn't find a mention of how that was achieved.
Quote on that thread from Gert on Aug 9:
As real engineers, yes, we have overclocked the ARM. But we can't tell you about it as this is an open website and one of our marketing managers might read it. They will immediately sell it with that spec.
Yes, we have overclocked the SDRAM but we raised the operating voltage beyond the safety limit. So if you want an ultra fast top-of-the-range Raspberry you can have, for a short while until it is "baked".