The Nexus S gets a slight bump to Android 2.3.3, which adds pretty cool NFC features, whereas the Nexus One finally catches up.
The new NFC features the NS gets allow it to turn the phone itself into a NFC broadcaster, whereas before it could only read. This way you can stick a URL or contact info for yourself in your phone and easily exchange it with other Nexus S devices. You can also program in NFC stickers from your NS phone too, by long-tapping.
Both phones will get this update over the air starting now, but may take a few weeks to finish rolling out. It also fixes the reboot issue people have been reporting.
[Are people seeing this here in AU? -ed]