Im not entirely sure what the max would be but Id imagine the computer would not post if it was to high(Ive been there with mine) but Id say yould be safe up to 1.4 with some good water cooling, maybe higher.
For these new 65nm chips the max voltage can vary. I would have to say that about 1.45 is the maximum voltage I would recommend for these providing your chip stays cool enough, this can vary depending on temperature of course
Heres where someone pushed thiers pretty far. The max Vcore the guy used was 1.55v with some realy good water cooling. Not sure of the actual limits, but maybe if you googled it you might find what your looking for, I think the Only CPU Ive ever heard of having a maxx voltage that killed it was the old northwood chips some call it SNDS, and it usualy happend around 1.7vcore. So like what many have said youve got alot more room to play with. Also the CPU is warranted up to 1.4 which is intels safe max limit.