Just because you CAN do something doesn't mean you SHOULD. This is definitely one of those cases. If you want your Nest to be running problem free on most systems (or any other smart thermostat for that matter), just assume you need the C.
[0] https://nest.com/support/article/When-Nest-needs-a-common-C-...
There are lots of reasons why most fancy thermostats need a lot of power. For example, a fancy screen is hungry, WiFi is an expensive protocol (e.g. as compared to ZWave), etc.. Compare the Lux screen to the Nest, and you'll see at least one reason why they think they can get away with batteries.
Another option, patented by Honeywell I think, is called "power stealing." Here's an interesting article on the subject: https://www.ecobee.com/2014/01/the-problem-with-power-steali....
Are you talking about re-purposing the G wire to become the C wire? I had to do that just recently, and I lost the ability to cycle the fan on by itself (without calling for heat or cooling). Not a big loss in my opinion, but my system is dead simple.
The most infuriating thing about it is that the previous HVAC installer cut the fifth wire off instead of sparing it out.
Wiring up a Nest isn't too bad, you just need the precious C wire. Most of these fancy thermostats require the C wire for electrical power, and many homes lack it. They have a work around, but it requires re-wiring and you lose some functionality (do not recommend).
PS - No idea why nobody offers an AC adapter as an alternative to the C wire. All the C does is provide electrical power anyway...
[0] The middle white thing: http://i.imgur.com/kNUPqvF.jpg