Gulftown, i5 upgradeability, sockets.
Alright, I did work at Intel, up until immediately before Lynnfield came out. (My last task was preparing a bunch of Intel DP55KG boards to send out to the press for Lynnfield reviews.)
Gulftown (according to public roadmaps) is the six core processor scheduled for early next year for socket 1366. The socket is not a limiting factor. It's purely a marketing choice to keep the higher-end socket higher-end. The lower-end socket 1156 will not be getting 6-core processors to keep differentiation.
Socket 1156 presently has four-core "Core i5" and "Core i7" processors available, the processors code-named Lynnfield. In the future, dual-core Core i5 and Core i3 will be available for that socket; including some with onboard graphics. (Motherboards for onboard graphics are not out yet, since the current processors don't support the onboard graphics.) Right now, this socket is limited to 95 W parts. Any future processor would likely also be a 95 W (or less) part. Numbers of cores has nothing directly to do with max power draw; Intel could make a slower-speed, higher-core-count chip easily. They've done it before. (For example, they have a socket 1156 Xeon that has four cores, but only draws 45 W. They do this by limiting the maximum quad-core speed. Using "Turbo Boost", the single-core speed is as fast as the 95 W parts.)
Eventually, a 'process improvement' (from 45 nm to 32 nm) will happen to these parts; with the onboard-graphics version coming first, and the four-core discrete parts coming later.
As to the upgradeability of the iMac; we won't know if they are socketed until they are delivered and someone takes it apart; but the very first Intel iMac used a socketed Core Duo; and was fully upgradeable to the later Core 2 Duo just fine. So short of soldering the processor down like they did later, Apple will likely do nothing to limit the upgradability. If the processor is socketed (as expected, since the just-released low-end Core 2 iMac is,) then you can safely buy the Core i5 version, knowing you can later upgrade to a socket 1156 Core i7. (Either the currently-available 860 or 870; or a future-available faster model.)
