According to Anandtech, the AM3 boards have the exact same socket...and use the same CPU's!
It has BOTH memory controllers built into the chip, since the physical differences are basically only on the memory socket side of the equation.
Not quite. You can't put an AM2 or AM2+ chip in an AM3 board. You can put an AM3 chip in an AM2+ board.
As to the socket differences.
Socket 939 is 939 pins Hypertransport 1.x up to 1.0 GHz
AM2 is 940 pins HyperTransport 2.0 up to 1.0 GHz
AM2+ is 940 pins HyperTransport 3.0 up to 2.6 GHz
AM3 is 938 pins HyperTransport 3.x HyperTransport up to 3.2 GHz
there are changes to the motherboards more than just name and the Hypertransport speeds, and memory types.
AM2+ gives you split power planes (which are present on AM3 as well). Putting a processor in a AM2 socket that has split power capability would probably mean it would use more power because CoolnQuiet couldn't power down the CPU cores and memory controller separately.
In addition to reducing the abilities of CoolnQuiet you'll restrict the newer CPU to older Hypertransport modes at lower clock speeds.
Even if you can put a newer processor in an old board it won't be as fast as putting the same processor on a newer board. The nice thing is that means AMD doesn't have to specifically make a crippled Celeron equivalent. You can take that shiny new AM3 chip made two years from now at a low cost and put it in an old motherboard and save the cost of buying a new motherboard + ram. The key for that will be to wait for the AM3 equivalent to the 4850e something mid speed with a lower TDP but with the L3 cache and maybe a third or fourth core (I'm fine with 2, 3, or 4 cores I'm not picky on that one).
Think about it down the road when a 780G board or equivalent is entry level and AM3 chips are common and you can pick a cheaper lower TDP AM3 part to put in your 780G AM2+ board. I don't know how many months or years that is away but that is what I'm looking towards. These high TDP first few months parts aren't the product SPCR types want.
This month it is 125W Phenom IIs, Next month there will be 95W Phenom IIs, sometime down the road there will be a lower TDP. It's when I see the TDP below 95W pop up that this will really get my attention.