One of the major ways a semiconductor manufacturer manages to make the most of its chips is through binning. This is the process by which chips are performance tested, then assigned a product line according to how they respond. Chips able to cope with high clock speeds with all cores running end up as premium product lines, while others will end up as models rated at lower speed grades, or with fewer cores.
Turning off cores
Binning is really important with new silicon designs. When chip architecture or process technology is new then there are usually more problem chips. One way of binning such chips involves turning off processor cores (or blocks of shaders on GPUs). Of course, as the technology matures there are usually less errors found and turning off cores becomes less of a necessity and more of a means of maintaining product lines.
The AMD lineup
In the case of AMD's Phenom CPUs dual and triple core models are quad cores with some disabled, while some newer quad core CPUs are actually six core models with two disabled. These disabled cores could well have defects that prevent them from operating properly, but some of the time these problems only affect one of the cores. And in some cases both cores might be fine and the product is binned because it cannot run at speed, or AMD needs to keep up a flow of certain processor models to the market.
Unlocking hidden cores
To this end both ASUS and MSI have announced that they have modified versions of AMD 890FX and 890GX based motherboards to unlock these hidden cores. These tweaks are done through the BIOS and require a moderate amount of technical ability (and patience). Much like overclocking, there is no guarantee that you will gain anything by unlocking the hidden cores - everything depends on just why your CPU ended up in a certain product line.
So while the technology in ASUS and MSI's BIOSes will indeed work, there is no guarantee as to what you will end up with. Think of it as a free performance boost rather than a guaranteed percentage improvement, despite what the motherboard manufacturers splash all over their websites.
At the moment the core unlocking technology is available on ASUS' M4 series of boards, including the AMD 890GX powered M4A89GTD series. MSI has the technology available on the 890GXM-G65, with it promised to appear on the rest of its AMD800 series boards through a BIOS update.