*** Edit 12 February***
It seems that AMD's AM3 Phenom II has a memory problem. The problem rears its ugly head when you have more than one DDR3-1333 (or greater) module per channel. If you have all four slots on your new AM3 board filled you could encounter "unreliable operation".
The Phenom II's memory controller has two 64-bit wide memory channels that when combined (in ganged mode) offer an effective 128-bit wide controller. On most mainboards this means you have four memory slots (2 slots per channel). For the time being AMD is not recommending using more than 2 modules (one per channel) for any DDR3 clocked at 1333 or above. If you have more than 2 modules the AMD quick fix is to change the speed to 1066 and adjust the timings to help overcome the speed loss.
It seems that the root cause of the issue may be linked to DRAM voltage.
It is possible that by slightly overvolting the RAM you may be able to overcome this issue. AMD is working on a more permanent fix already.
***I spoke with Damon Muzny at AMD about this and it is not actually a problem. This behavoir is actually by design. The reason is that AMD could not predict the behavoir off all DDR3-1333 memory. They found that with certain lower quality memory modules and all four slots populated (2 modules per channel) they encountered instability. They were faced with a choice; either they could drop listing support for DDR3 - 1333 or they could design the system to down clock the memory to 1066 and recommend it to everyone using 2 modules per channel.
You have probably guessesd what they chose.
In all honesty you can usually get more performance from a lower clock and tighter latency than a high clock and loose latency. Or you could always bump up the memory voltage and bring stability back that way.