A lot has been said on this subject. There have been lots of rumours about the chips degrading under voltages higher than x volts (The line is generally considered too be around 1.4V)
Whilst there may be a wisp of truth to this, many people have been running their chips at >1.4V since release with zero problems. I have been running 1.368V since release and around a month ago I increased my overclock using 1.44V, no problems so far.
IMO, the degradation is not down to the CPU Vcore, but to the FSB or VVT voltage. I don't know what the limit is here, but I can say that I'm running 4.4GHz with 1.1V. I've seen some people using 1.4V for no apparent reason. So don't increase the FSB voltage unless you are absolutely sure that it's what is holding you back.
AMD's 45nm SOI process seems to be much more voltage tolerant with no reported degradation even with lots of people running at >1.5V