Well 1156 is officially dead as Sandy Bridge is moving to 1155.
Hexacore won't come down in price - Extreme Editions never do (so 980X and 990X will stay as they are), and the i7 970 doesn't have any competition to drive it down. By the time we see hexacore Sandy Bridge, socket 1366 will be very long in the tooth and have something to replace it anyway.
To be entirely honest, buying a computer with any desire or intent to "upgrade it in the future" is a waste of time and false economy IMO - everything moves so fast these days there's little scope to upgrade. Intel don't even bother with any pretence (and that being a good or bad thing is left to your discretion), and even though AMD to try and extend their platforms to cover a few CPU generations, eventually they have to give up and just start again (upcoming Bulldozer is a good example).
Just buy the very best you can afford now, and if you have the ability to do an upgrade in the future then bonus. With that, 1366 does have the option of dropping in a hexacore if you're only going quad for now. The only way you're going to get any real longevity in a computer is to make the investment now and plan your spec and purchases for what you're likely to be doing.
For instance, I'm going 980X, 12GB Dominator GT, Rampage III Extreme. Do I need 6 cores? Not really - I'd be fine with 4. Do I need a top-end motherboard? Not really, but it's a great pairing for some serious overclocking. Do I need such robust RAM? I will do when I start overclocking. But as the software I use starts to embrace multiple cores properly I'm getting new benefits for free. As more CPU grunt is required, I can start overclocking the CPU to compensate, and the top-end motherboard and RAM will help in this area. You start getting more life out of the system, extending your initial purchase.
Spend £1,500 every 2 years, or make a £3,000 investment now that will last me 8-10?