Later this year Intel will introduce a line of new quad-core processors which will be pin-compatible with its current Core 2 line. Anandtech have taken engineering samples of this chip and successfully upgraded a current generation quad-core Mac Pro to run with eight cores.
I can't say I'm too surprised this works, after all it is pin compatible, that's the point. But it's good that it works so easily, not only because current Mac Pro owners have an upgrade path, but also because the rest of us can look forward to off the shelf eight core Mac Pros in the (near?) future.