Google started pushing the latest version of Android for Nexus S a few weeks ago. If you have not yet recieved the update for your Nexus S, you can upgrade to the latest version manually. It's interesting as my wife got an update yesterday but I did not get one so I updated mine manually. Here are simple steps to manually update your phone to Android 2.3.6. As usual we warn you that manual upgrade may break your phone, so do it at your own risk. Muktware will not be responsible for any damage caused to your phone.

Warning: Make sure your phone is fully charged. Back-up all the data/image to your Linux PC in case something goes wrong you won't lose your images or data. This update is ONLY for Nexus S phones.

Instructions
1. Download the zip file of the update from this link. Rename the file as update.zip.
2. Connect your Nexus S to your Ubuntu/Linux PC.
3. Drag and drop the update.zip file to the root directory of your Nexus S.
4. Disconnect your phone from the Ubuntu PC.
5. Restart your phone and when its about to reboot hold the Power and Volume+ key. This will boot your phone into set-up mode.
6. Use the volume key and select 'Recovery'

7. Once it boots into recovery mode you will see an 'Exclaimation' icon. Click the Power and Volume up buttons. This will give you an option to chose how you want to recover. Use volume key and choose the update.zip that you copied to the phone. This will start the update. Reboot the phone.

8. If everything goes well you will be running Android 2.3.6 on your phone.