If your Nexus S is stuck on 2.3.1 you can't upgrade to 2.3.3 without first going through 2.3.2. But FINDING a a link that works for the 2.3.2 download isn't as easy as the 2.3.3 download. I found 2.3.2 on MediaFire (via Software2tech).
I have no idea why Google can't seamlessly push out upgrades to its flock of Androids. But mine's finally upgraded.
Instructions on how to upgrade are everywhere. Here's one set of instructions.