I have a gas fireplace tied to a zwave switch downstairs. I would like to have it turn on automatically when the temp downstairs is some threshold below the temperature upstairs and turn off when temps equalize. Is there an app for that? This would be a great use case for space heaters also.
There are some apps that look like they might do what you want in the official smart set up apps. These are the ones that are already available for quick installation from your smartthings mobile app.
Take a look and see if any of these fit. You would need to have a Smartthings connected temperature sensor in both zones. In particular, “keep me cozy two” (#3 in the Green Living section) might work.
As far as devices, you’re likely okay with your gas heater, but underwriters laboratories and most fire departments recommend never putting a space heater on an unattended mode. Those things just cause too many fires to run remotely. But there are many other good use cases for a zone to zone comparison. 
I’ll repeat that running temperature control over internet is bananas.
I take you guys at your points. Some follow up questions:
If the internet were to go down, would all my smart things trigger events stop functioning even if they only referred to local variables (like zone temp)?? I can see how this would be an issue with any attempt at temperature control… 
Do you think such a use case would be sufficiently safe it only came on when someone was home and detected motion in the room?
With the v1 hub, your events would not fire. When they release the v2 hub, that one should fire events without internet
Good to know. Sounds like I should avoid a use case like this until the v2 hub is released and i jave one.
What device did you use to trigger the fireplace?
I have an on/off toggle switch separate from power (may have some current in it) that I tried wiring with a ZFM-80 but fried it.
Sounds like you tried to wire the low-voltage contacts into the “line” lead. You cannot do this. You have to supply it with a 120V AC source that is separate from the low-voltage line that switches the fireplace. Perhaps there is an outlet you can run line from that is nearby? I have a standard switch right next to my fireplace switch that controls the fan… Since I don’t have a fan, I just hijacked it for my relay power source.