There is no such thing as "future proofing", that term really needs to die. Motherboards hardly stray from their price base (~$100), they're a critical part of the system and they see improvements just as much (if not moreso) than any other part of the computer. If you're willing to invest in CPU upgrades down the road you should really consider motherboards into that equation.
Its one of the quirks with overclocking. You might need to buy a more expensive motherboard than your avg joe, or perhaps a new one all together, but finding the right one and the right chip can save you from spending hundreds on a top of the line CPU - as you can get that performance from a 3000+ or Opty 165.
I guess I just don't understand the complaint about socket longevity, because it isn't necessarily cost that I can see as the biggest let down for me, it would be the hassle of installing the new motherboard. Otherwise I don't see how it can be so hard to accept that performance improvements might require platform improvements...
M2 migth not make sense based on performance speculation what with DDR2's dissadvantages, but DDR2 is cheaper to manufacture and is easier to package in larger quantities. We might not have faster ram, but we'll have more GB of it and for cheaper to boot. 4GB and beyond won't be out of reach as we'll have larger capacity sticks at reasonable prices (finally, a good reason to have a 64bit chip) - you won't be able to get that with s939.