I think the reason for the short life span of the 1156 is that Intel changed their roadmap due to economic recession/lack of competition/piles of 775's.
Lynnfield was supposed to show up earlier, and a 32 nm 4C successor was planned, then they changed their plans and messed up the tick-tock strategy.
To me it's quite obvious that those late change of plans wouldn't change the plans of SB back then, IE maing it backwards compatible.

Sandy bridge is a fresh start and I guess it's too early to tell if the 1155 will work with Ivy Bridge, but I think it's likely.