With the introduction of Cool'n'Quiet 2.0 , AMD is taking power saving a step further than Intel's SpeedStep. Each CPU core is able to reduce its frequency if its full computing power is not needed. In this power saving mode, each core can run as low as 1200 MHz when idle.
This is a change compared to the Athlon 64, which was able to run as low as 1000 MHz using a 5x multiplier. Phenom, on the other hand, uses 6x as its minimum multiplier, making the lowest idle frequency 1200 MHz. The reason for this behavior can be found in the Hypertransport 3.0 interface. It runs at a frequency of 1200 MHz, and the processor's speed cannot be lower than the Hypertransport speed.
Since the CPU is downward compatible, it should be possible to revert to Hypertransport 2.0 or 1.0 in order to achieve an idle (power-saving) frequency of 1000 MHz or 800 MHz, respectively. Sadly, we didn't find such an option in the BIOS. Perhaps a resourceful enthusiast or programmer will write a clever tool on his or her own that enables this on-the-fly switching...?