According to the post at Korean website Playwares.com, the fourth core which is disabled isn't locked and can be enabled in BIOS, thus making it a quad-core CPU.

Screenshots look quite real, and we can't think of any reason why they shouldn't be, but you still might want to take this one with a grain of salt as this might be an isolated case, or some weird kind of Photoshop prank.

Guys tried this with Biostar's TA790GX motherboard, but according to the post, the same thing can be done with any other 790GX + SB700 motherboard. According to the screenshots, all you have to do is to modify "Advanced Clock Calibration" to Auto and your tri-core should become a quad-core. Guys at Playwares.com tried it with AMD's Phenom II X3 710 processors, that has for some weird reason became the AMD Phenom II X4 10 processor, at least according to BIOS post screen.

CPU-Z and Windows both recognized the fourth core, and these guys even managed to do a Prime95 and 3DMark06 CPU tests. The fourth core brings additional 1000 points in 3DMark06 CPU test which is a really nice score considering that you only payed for a tri-core CPU.

The only problem is that this might not be possible on all AMD's X3 processors as in some cases it might not be so stable. After all, AMD does make these X3's by disabling one core that is unstable.

Here are some of the screenshots and you can find the rest here.