Nexus S will ship with support for "fastboot oem unlock", allowing for reflashing of the system software "out of the box", like Nexus One.
Something that may interest this community is that the NDK (native development kit) for Gingerbread now supports native apps (intended to simplify mobile gaming ports, etc) -- providing: libc, libm, libz, opengl|ES, opensl|ES, input/events/sensors, app lifecycle management, etc. JNI is available to access various higher level Android APIs as necessary.
Platform sources should ship at or shortly after commercial launch of Nexus S.
Kernel git repository (2.6.35 + android + s5pc111/nexus-s) will be available at or shortly before launch.