The Lynnfield LGA1156 Core i5 & i7 are the processors that are designed for the mainstream market and will be the logical successor from the Quad Core LGA775 Core 2 processors such as the legendary Q6600.
Unfortunately Intel has caused quite a bit of confusion for consumers, enthusiasts, and manufactures by releasing the new CPUs on an entirely new socket but then rather than just have i5 CPUs on LGA1156 they have included i7 CPUs on this socket as well.
So you basically have i7 CPUs on 2 completely separate sockets and it appears the main difference is the fact that no LGA1156 CPUs use Hyper Threading and they have a less sophisticated memory controller that can only support dual-channel DDR3.
For the time being Intel have released 3 CPUs on the new socket and this includes the i5-750, i7-860 and i7-870.
The i5-750 seems to be the best choice out of the three as it is clocked at 2.5Ghz and is priced competitively with the previous generation LGA775 Quad Cores.
Over the next few months you will likely see many big name brands such as Dell and HP release full systems based on this new socket/CPU.