No, the OTA IS actually out.
Jean-Baptiste Queru (who works at Google on AOSP) confirmed it on Twitter. Sprint also has representatives confirming that people HAVE received the update.
However, the update is most likely being given out slowly so as to not overload the servers.
Remember, not everyone who has a Nexus S 4G will go OMG MY PHONE UPDATED TO ICE CREAM SANDWICH because most people don't even know what "Ice Cream Sandwich" even is. All they'll know is "Huh, my phone looks different. Cool." and then go on about their day. There are more non-technical people out there who have this phone than you think and they won't make a big deal about it nor will they rush to these forums or XDA to figure out how to share the OTA.
Not to mention there's already someone on Sprint's forums who has the OTA but he didn't get the URL either, he simply updated his phone and that's that.
Now, as for how to get the URL (courtesy of Maximilian Mary on XDA):
If you get the update notification, say "not now" when it prompts you. Get the filename from /cache, and put this in front of it to get the full URL:
http://android.clients.google.com/packages/ota/google_crespo4g/.
You don't HAVE to do this. You can simply update it and not help anyone else get it. This is just to be nice and make sure others get it as well.
(EDIT: I just remembered something. Once you do upgrade to Ice Cream Sandwich, you will no longer be able to do the update.zip method for OTAs. You'll have to flash the factory images or simply wait for the OTA itself. If you do try the update.zip method, you'll get "Error Status 7", which is what a lot of people with the Nexus S ran into when they tried to update 4.0.3 to 4.0.4.)