I have been trying to crack the room presence detection (for home automation) problem for more than a year now. And for now the only reasonable (cheap) solution seemed the IR sensor approach - use it to count how many people are in the room. And this usualy works pretty well but it has problems when people are too close together or just stand in front of a sensor.
RFID for the most part was unusable because of the short range (few cm) and a quite expensive hardware. But after some browsing I found the solution - active RFID. And it is cheap too. I found a seller on ebay that sells RFID receivers and transmitter tags for a realy realy low price: http://stores.ebay.com/Ananiah-Electronics/_i.html?_fsub=6029549 . The detection range is about 8m and apparently it works realy well (I found talks about it on some forum). The only problem is that the tags are not that cheap (but not expensive either) and that you have to wear them.
If I wanted to interface a reader like this one: http://www.ebay.com/itm/RF9315R-Active-RFID-Receiver-Module-RSSI-/250396932886?pt=LH_DefaultDomain_0&hash=item3a4cd1fb16 to the arduino (without computer), what would be a better choice - USB one or an RS232? Or would I need level translators on both of them? In this case it is proabably better to go with USB and use the computer for processing.