I mentioned before that even though Haswell will be a new socket (1150) and sockets usually last 2 generations (meaning Broadwell would also use socket 1150), that it was unlikely they'd be compatible with each other due to Broadwell's multi-chip package design.

That no longer matters. Tick-tock is being abandoned. Word out now is that Haswell will be the final CPU from Intel that you can upgrade. Broadwell will use BGA packaging, meaning the CPU will be directly soldered to the motherboard. This has efficiency and heat benefits, but you can see how upgrading will require replacing the entire motherboard. How this will effect motherboard manufacturers and options included on motherboards is not yet known.

"Extreme" processors and Xeons are not expected to be effected.