I wanted to keep mine below 1.3v so that it wouldn't degrade the chip very quickly. I remember reading an article where a Wolfsdale Core 2 Duo burned out after only 6 months at 1.4v, and they recommended <1.3v or it could degrade the chip in less than a year. I don't know if this holds for the Core i7, but I didn't want to take the chance.
So, I'm sitting pretty at 4.2Ghz at 1.2975v, one step below 1.3v on my P6T. It's perfectly stable running everything I've tried so far. (I started at 4.0Ghz and tried going down as low as 1.25v and stability testing, raising it one notch at each failure, and eventually evened out at 1.2975v at 4Ghz, then ramped up the speed until it became unstable again, dropped it back down one notch, then stability tested it for 24 hours. You see my results.)