I know everyone has done this before. RFID and arduino that is. But looking at the example code it looks like the antenna is always in receive mode. I am not sure how this affects the life of the chip / reader but I thought of adding a way to detect human presence before activating the receiver.
I found some little IR heat detector (here: www.allelectronics.com/make-a-store/item/IRD-10/INFRARED-... and tossed together some analog read code and viola. Now when the IR detector detects over a certain variable heat temp it activates the RFID reader.
I will post the code and write up on my blog.