It's obviously in response to the upcoming C2D price drops (later this month) and the recent release of intel's bargain basement priced E2100 series chips.
They aren't really introducing a new socket, it's more of an upgrade to the current AM2.
AM2+ procs will be backwards compatable with AM2 boards, the older boards just won't be able to offer the full range of features (such as separate clocking of NB speeds from CPU speeds).
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