Mark's assumption was based around an $80 motherboard. People who want more feature-rich motherboards will realize that the AM3 cost advantage quickly diminishes as soon as you start considering the higher tier boards (790FX and the like, but then again, you do get dual x16 PCI-E lanes here...). But for those that need a no frills board, then yeah, total system costs for AMD will be a lot cheaper.
Personally, I just think that hovering around the $200 price point is too close to the i5. I think the entire Phenom line needs to be bumped down around 10% in price to be more competitive. I know it's stupid to have your flagship selling for so low, but hey, I think that's what it's worth.