Looks like it will be
January 9th for the mainstream (LGA 1155) quad cores

Sandy Bridge's "mainstream" level will be a mix of true quad cores (i7, most of i5) and dual cores (some i5, i3 and pentium), however the dual cores seem to be coming out on the 20th February.
The "enthusiast" level Sandy Bridge platforms (LGA 2011 and LGA 1356) are expected Q3 and Q4 2011 - with no firm info on specific dates. These look like they will range from 2 to 8 physical cores, all with hyperthreading enabled. The main difference between s1356 and s2011 (apart from different CPUs being installed) is that s1356 will have one QPI connection (s2011 will have 2), s1356 will have three memory channels (s2011 will have four) and s1356 will have 24 PCI-E v3 lanes (s2011 will have 40).