I have watched several unboxing videos for the Nexus S and I noticed a user guide was not included in the packaging. I know there is an Android 2.3 user guide online but it is not specific to the device.
I know my nexus one had a PDF to download so I assume google will release it on Thursday after the phone is released. Who knows it might be posted online already.
I'd be willing to bet Google will have it up when the 16th comes around in time for the people who bought the phone to have something to refer to. We'll soon find out.
I thought it was on the Nexus S site, but that's an Android 2.3 manual. It does say "Coming soon from Samsung" so I expect there will be one available on General release.