Sandy Bridge may or may not be a drop in replacement for either of those sockets. Intel has announced no such compatibility and their track record on that issue is not promising.
From what I have seen its a lower pin count (1356 for example) which means it could still be a drop in like Phenom II AM3 was. As for their "track record" that was more based on the mobo vendor than Intel. If you went and bought a cheap mobo, it probably couldn't support the TDP or voltage required. But Asus has a 945 based chipset that supports all of the Core 2 Duos and Quads. Its just like with phenom and Phenom II. The BIOS needs to be updated and the caps need to be able to handle the TDP. Some people had AM2+ mobo with only a 95w TDP max so they couldn't get anything beyond a Phenom II 920.
So far the DT is still LGA1156 with no pin count change, only high end. So thats more of a possible drop in than high end will be. Of course things can change.
For anything I would expect 22nm to need a new socket.
The rumour mill has it that Intel are working on yet another new socket for Sandy Bridge. So much for backwards compatibility!
That socket from what I have heard I think is specifically designed for the CPUs with GPUs that were based on larrabee at the time so that might change since Larrabee is currently stopped for the consumer market.
As for sockets, I would rather have a new socket that works properly than a old one that could cause problems. Plus most of the time when a new socket comes out or chipset, there are better features. A lot of people will put Thuban into a AM2+ but then their mobo will have other bottlenecks such as SATA 3.0 and USB2.0 when if they got a new AM3 with the 890 chipset, they would have less of a bottleneck there.