NFC functionality in Google Nexus S? How physically close will a tag need to be for the phone to detect a NFC card? (10cm or can it greater)

Also can the phone present itself as a NFC tag to other phones? If this is possible is the range greater as the NFC transmitter is powered?
The specs on the NFC Controller chip in the Nexus S are the following:
http://www.nxp.com/acrobat_download2/literature/9397/75016890.pdf

To answer your first question, as advertised in the above PDF, the NFC Controller chip inside the Nexus S has an operating distance of about 10 centimeters.
However, in practice (specific to the NFC controller in the Nexus S that is), it's limited to about 4 centimeters. You practically have to hold the card to the back of the phone in order for it to be able to read the card. 10 centimeters is massively over-exaggerated, 4 centimeters is the farthest I could go with any card. Distance is greatly dependant on the quality of the RF transmitter, and receiver on the other end, so this distance may very well increase under different conditions. If you have specialized equipment, it's possible to pick up any NFC RF communication up to a distance of about 10 meters, but don't expect NFC implementations in phones to go any farther then 10 centimeters.

For your second question. Again, as advertised in the above PDF, card emulation (the ability to present oneself as a NFC tag) is supported for the following standards: ISO 14443 –A-B-B’, MIFARE, FeliCa RF.
To my knowledge, these are all the standards one should (currently) care about. 

However, card emulation is not supported by the Android SDK, and I'm not sure whether or not it will be supported in future versions, or even if Android Devs are working on such functionality. It would, however, be the logical next step for the Android SDK to support.