Sprint is not going to put it out. Google is. The Nexus line of devices are Google experience devices, and carriers and manufacturers don't put out the update; Google does.
And yes, Google is going to put one out. The binaries have already been released, from which developers have put out their own builds for those who want to brave the waters (voiding their warranty) and root their phones and install one of those ICS roms.
There are at least 3 other threads already discussing this, but what the heck...why not one more, right?
Sprint will not be putting out the update, Google will. Right now Google has paused the Ice Cream Sandwich update for the GSM Nexus S because of update problems in specific regions. Considering that the Nexus S 4G has a much smaller install base, than the GSM Nexus S, we probably won't get the update until next month, though I would be really happy to see the update before New Years.
If you are impatient and simply can't wait, just root your phone and install a custom ROM from XDA like the oICS ROM for the NS4G. It's a really good ROM and has everything working, including the Face Unlock. I was running that ROM before I unrooted and went back to stock (hoping to get the official one before Christmas).