Do wing warmers exist?
The standard solution for keeping aircraft parts "above icing temperature" when on the ground is, as was pointed out in the comments, a nice heated hangar.
This has the secondary benefit of keeping all the other components at a warmer temperature where they're more amenable to operating (gyros spin up better, wheel bearings rotate more easily, etc.)

Once outside of a hangar deicing fluid is used, and provides "anti-icing" protection for a specific period (the fluid's "holdover time") - we discussed that in more detail on this question.
Obviously once the aircraft has been exposed to icing conditions or snowfall for a period exceeding that holdover time it must be de-iced again as there is a chance for ice to begin accumulating.

There is also the potential to use the aircraft's ice protection systems (in particular electro-thermal systems such as used in the Boeing 787 which heat the wing in the manner you allude to in your question), though I'm not aware if any systems are currently approved to be used in this way.
One of the key challenges there would be verifying that the wing surface is indeed free of ice, snow, and frost - that's easy to do from the deicing truck (particularly since deicing fluid is tinted so you can see where it has been applied), but on a typical passenger jet the pilots can't see the wing from the cockpit, and poor visibility from blowing snow, fog, etc. may make the visual determination difficult from the cabin as well.