Baby monitors are pretty useless - they only tell you when the bub is making noise and if they're making noise, they're still alive! :) It's when the little buggers are quiet you have to worry.
There are mattress pads that can monitor their breathing but if they roll off the pad you'd probably have a minor coronary as you rushed into the room only to find the tot curled up at the far end of the cot.
A little pair of smart pj's though ... what a great idea! Bub stops breathing - alarm goes off! I'd pay $100 for that!