I'm attempting to build an Arduino-based device that will use NFC to know when my phone is close to it. My phone is a Nexus S, equipped with NFC. I'm looking for what I believe is called an NFC Transceiver, to be able to pair with my phone via NFC. I've found plenty of technical documents on this component, but can't seem to find anywhere that the general public can buy them in small quantities (namely 1 to 5).
The term you're probably missing is 'mifare', which is the product name for the basic, unencrypted NFC tags. Sparkfun sell this transceiver, which works with this Arduino shield. I have one, and though I haven't tried using it with my Android in 'tag mode', it ought to work - you'd have to try it to see.
I would suggest Adafruit's NXP PN532 breakout board: http://www.adafruit.com/products/364 It is a very reasonable price and the NXP chip allows for all three of the NFC modes: read tags, emulate tags and do peer-to-peer between NFC devices.
The only downside is that they're frequently out of stock and sell out very quickly when boards do arrive.
You need to realise that your Google Nexus S phone will have to run in tag (or Card Emulation mode). As of today, the official Android on the Nexus S does not provide the required APIs to enable Card Emulation mode. (Latest Android running on Nexus S is ICS 4.0.3).
You will need to hack the phone, and flash a custom ROM on the system.