It would have to be standard with the car - if not, the only people who would have them would be the hyper-vigilent, who'd not likely leave the kid in the car in the first place. But, still a damn good idea.
Actually, see post 153 or thereabouts. There's a way for cars with car alarms to have a baby seat monitor set off the car alarm if the car key goes out of range.
Just like you can buy and program a new car alarm remote, a baby seat alert could be programmed to be able to set off the car alarm using the "panic" button code.