Looking at it from an engineers perspective you would need something that could tell the difference between a passenger in the seat and a baby in the seat. Using just a simple pressure sensor wouldn't work because if someone was left sitting in the car, like a 5yr old with others in the car while someone went inside a store it would be hard to determine what the situation was.
What could be done is to combine the pressure sensor with a temperature sensor. Weight on the seat and the temperature is outside temps safe for humans then the alarm is triggered. Would be very simple to implement on a car. I could do it with $20 in parts from my parts drawer it is so simple.
Curious I checked what it would cost to make a DIY version for someone without any supplies at all:
Force Sensitive Resistor 0.5" $6.95
http://www.sparkfun.com/commerce/product_info.php?products_id=9375
One Wire Digital Temperature Sensor - DS18B20 $4.25
http://www.sparkfun.com/commerce/product_info.php?products_id=245
Arduino Pro 328 - 5V/16MHz $19.95
http://www.sparkfun.com/commerce/product_info.php?products_id=9219
Relay SPDT Sealed - Mini $1.95
http://www.sparkfun.com/commerce/product_info.php?products_id=524
$33 for a DIY version.
resistor determines weight based on pressure and connects to arduino board. Temp sensor can read -30 F to 120F connects to arduino. Relay connects to arduino and when triggered closes wire to cars alarm system setting it off.