I'm having the same problem with the same CPU and motherboard.
I'm also using

-OCZ Gold 2GB (2 x 1GB) 240-Pin DDR2 SDRAM Unbuffered DDR2 800 (PC2 6400) Dual Channel Kit System Memory
-ASPIRE ATX-AS520W BLACK ATX 520W Power Supply 115/230 V CB IEC 950/ TUV EN 60950/ UL 1950/ CSA 950
-XFX PV-T71G-UDE7 GeForce 7900 GT EXTREME (520MHz) 256MB 256-bit GDDR3 PCI Express x16 Video Card
-Thermaltake 745 Liquid Cooling Kit

I also get 1.5 volts at regular clock speed.

I've tried overclocking it, and ran some stress tests at 3.6 and 3,8 ghz. It got through some, but it was never completely stable, and it couldn't handle playing Prey for more than 30 minutes. I checked the heat everywhere, and the motherboard didn't go above 45C, the CPU was below 55C, and the GPU below 70C, so they all should havebeen fine. The only thing I can think of was that something wrong with the voltage, becase it's 1.5V at 2.66ghz
Should I return it? Is there some way to fix this?

I've got a similar set up myself, using a ASUS P5W DH. I get a core voltage of 1.33 but understand that different chips can require different voltages based on the quality of the chip. It's a overclockers lottery when buying a chip, and toms hardware point out on their overclocking the 805 article that not all processors are the same, and even when they are, you still may not achieve the say high clocks speedes due to variations in the processor. Looks like you guys just got unlucky on this one.

If your concerened it's the motherboard doing something, just go into the BIOS and reset the settings to default, just to make sure.

Welll, thanks for that analysis...I think I'll try and get an RMA then, since it's operating over-voltage at regular clock speeds, but if not, I'll probably just end up buying the E6600 in a few months, since I've heard that it sounds like a very powerful chip for a decent price. It will work with the Asus Motherboard I've got, right?

Regarding reseting the motherboard to default, I've tried that several times, and I don't think the problem is the NOS, because I did manual over-clocking, which shouldn't turn on the NOS feature.

Oh, and you don't need to disable the NOS feature, if you want it on you have to enable it by going to the "Advanced" Tab in the BIOS menu, then go to "Jumper-Free Settings," and then the first option is what type of overclocking you want, and it's either disabled, NOS, or Manual, or something like that. But by default NOS is off, so unless you've turned it on that shouldn't be a problem.

Oliver, I have seen similar voltages with my Asus P5LD2 and Pentium D 820. The default MB settings set the VCore to something near 1.5V. I have entered in the Bios and set the VCore to 1.225 V (the lowest setting), and the CPU is running perfectly stable (always resulting in a VCore of 1.2 or higher, even under load).

I don't think you should worry about the 1.5 V, your CPU will probably not suffer. But lowering your VCore as low as possible (while running stable) will reduce the temperature of the CPU.

Well, I wasn't using CPU-Z, I was using the motherboard probe and some other programs, but I'm now pretty sure the issue was a problem with my power supply, because I switched to a different one, and the voltage was regular, and I was able to OC to 4.0, and run all the benchmarks I wanted, and it's perfectly stable and happy. (there was also a little problem of artifacting with my video card, but after reading comments on newegg I set it to auto-overclock, and now all the artifacts went away too). So unless something doesn't work in the next week, the new power supply seems to have solved all my problems.