For as long as multi-core processors have existed, CPU manufacturers have been disabling and under-clocking the physical cores of higher-end units which proved unable to meet their intended design specifications. Sometimes, the failures are subtle enough that by applying more conservative settings, chip makers can re-use this "defective" silicon and save on production costs. These downgraded processors ultimately wind up on shelves as re-labeled products, often times with lower advertised speeds, fewer cores and most importantly, lower prices.
The potential for enthusiasts is ripe. Code named "Zosma", the new quad-core CPU is very closely based upon its upscale, six-core counterpart known as Thuban. By enabling two cores, a user can easily unlock Thuban-like potential, Turbo CORE and all. However, not all processors are created equal and in the true spirit of over-clocking, the mileage enthusiasts get will most certainly vary.
At least initially, not all new Phenom II boards will support this feature. Models said to sport the new options include the M4A89GTD series for Asus, 89FXA-UD7 for Gigabyte and 890GXM-G65 for MSI with the promise of many more to come.