Is there a device out there that can monitor ambient room temperature and kill power on a 110-120v ac circuit if a certain temp is reached? I'd assume some sort of temp sensor and relay is involved. Thanks for any insight.

Yes - it is called a thermal shutoff switch. The ones I've seen have a 105 F shutoff (which does work). The problem is the only one i've found already wired (I am not an electrician) is into a flimsy household extension cord (though it should still work) from sunlightsheds.com. If you are an electrician (or know one you trust) you can buy just the thermal breaker switch and wire it into any cord you want.

It is on the bottom of this page (the picture looks like a heavy-duty cord but it isn't!):
Instruments

I've seen 120V Attic fans pre-wired with thermostats to turn on and off within a temp range.

I'm sure any decent hardware store would either have a replacement thermostat, or can order it.

Otherwise, I know that you can buy thermal fuses to wire in place. Works just like a electrical fuse, when it gets above a certain temp, it trips and cuts the power. Those however cannot be reset, just replaced.

__________________
Even monkeys fall from trees -- Japanese proverb
Check out my Super Steath Perpetual Harvest

I'm thinking about ravaging a space heater, some have adjustable thermal shut offs which I could easily integrate. It seems this may be cheaper in the end. If anyone has any cost effective ways that are better, I'm all ears.

To be honest, if you are checking on your plants regularly and have a indoor/outdoor thermometer - i doubt you'd really need an auto shutoff.

I mean paper doesn't ignite until it reaches over 400 degree F - F 451 according to Bradbury anyway

That's not to say safety isn't important but unless you are using several 1000w HIDs and growing in a cardboard box i think you should be safe. Or was this to prevent plant heat damage and not to primarily to prevent fire?

Preventing fire is the only reason. 400w hps in a 32"x32"x4' box. light is vented, box too. Should a fan die, I don't need the fire department coming to my house. So I decided to research a thermal cutoff if you will, for safety's sake. And to protect the babies too i suppose but the #1 goal is fire prevention.

I was also investigating putting a shut-off in my box too for the very reasons you mention (which is why i knew about the thermal shut-off switch). But what I ended up doing was running the system with no fans and the lights on. I kept a close eye on it while i was running the experiment of course. I did this during very warm days for a week (24x7 1000w HID) and the box temperature leveled off at around 98F and never got hotter than that (and was usually less than 95 F).

So i know even if the fans break (and everything else continues to run) i'll level off at 98 F during the hot days. And that makes me not worry too much about it anymore. So maybe an experiment can relieve your concern.

Ace - Ace Tree: Heating & Cooling: Thermostats & Registers: Thermostats: Lux Products® Programmable Thermostat (WIN100)

considering this for peace of mind, at least, b/c I'm paranoid about being safe.