If you don't need it to be accurate here is a cheap and easy way of adding it to your next Arduino project.

For this project I will use a pyroelectric sensor from a PIR motion detector. Of coarse the temperature reading is not going to be very accurate but if you need accuracy then you will need something better then a $2 PIR sensor. 

There are a few things to note about PIR sensors. 

Ok lets start

Modding the sensor is done with a small bit of foil tape. Because the sensor has a left&right sides that increase and decrease the output depending upon which side detects the most heat we will cover one side. I used foil tape as the metal is a good IR reflector and a good conductor of heat and will hopefully disperse the heat. I found it was better to block the side the lowers the voltage, but you can do ether.

   

Wiring the sensor is easy, They only have 3 pins (Drain/Vcc, Source/Out, Ground). To wire to an Arduino you need to connect up as below

Above is an extract from datasheet with my notes on it.

I didn't have a 47k so I used a 20k, still works well.

The Code to get the sensor value.

                        sensorValue = analogRead(sensorPin);

What did you expect it to be more complex? You can use most of the Arduino demo sketches to use the sensor. The only thing to note it that analogRead works better if you put a delay in front of it to let things settle first. 

Putting to use 

If you want something a little more complex you could try mounting the sensor on servos and use it to track people and their appendages ;) and oh yes that is a wireless camera on top.
You could even use it to map the surrounding environment.

In the video below the head does a sweep reading the PIR sensor values then returns to the strongest point with the lights, then repeats. The values are outputted to the computer if connected.  

The next step for this that I am working on is to process the data computer side to create a 2d PIR image, almost like Thermo graph but without the stupidly expensive price tag. 

You can get the sensors from freak labs store like I did for $2 each
http://www.freaklabsstore.com/index.php?main_page=product_info&cPath=36&products_id=128

The Datasheet is also downloadable from the product page above.

You can also get the sensors from other online stores like (I'm not plugging them its just were I shop) 
http://www.littlebirdelectronics.com