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]