While there have been numerous Nexus S 4G ICS builds built either from source or ported from the Nexus S source, we have yet to see an official ICS build for the Sprint Nexus S 4G. That changes now, as XDA member mrxkills has posted the first leaked ICS firmware. Bearing the firmware version IMM26, the zipped up ODIN package contains a bootloader and updated modem as well. The XDA post also contains a ClockworkMod recovery zip for the ROM and one for the kernel, sans bootloader and modem.
qbking77 has also put together a very informative video detailing how to install this via ODIN.