A new socket, package, and chipset
Obviously, with all of the changes built into the Core i7, retaining compatibility with Intel's existing LGA775 socket was out of the question. In its place, Intel has introduced the new LGA1366-style socket with, tada, more pins. Betcha can't guess how many.
Anyhow, this new chip socket and package demands a few pictures, so here you are...
As you can see, the Core i7's new package is relatively large, as these things go. I'd expect a different, smaller socket and package for future mainstream Core i7 derivates.