Dunnington, alas, will not work in the current Mac Pro systemboard, so we won't see it in an Apple product.
And it looks like Intel will be shipping the single-CPU Nehalem solutions first. September will be a "paper launch" with actual product not becoming available until October.
So Gainestown might not be available before 2009 and as such we might not see a Nehalem Mac Pro at MacWorld in January.