One of Intel's directors has told Fudzilla that Nehalem is pin compatible to the current Core 2 Duo generation but due technological differences it simply won't work in any  of the existing boards.

He said that you will need a next generation chipset, Catanga is the big codename and their internal was Ohio or Ioha.

It is mainly because of the memory controller on the CPU and new interconnections. The same chap also said that they are very flexible on the CPU.