Hi!!
It might sound like a very noob question, but anyway, here it goes:
Im going to power the arduino with an LiPo battery from a RC helicopter, the voltaje of that battery is 11.1v, but when its fully charged it reads about 12.48v - 12.7v, is it safe to power up the arduino, with this battery?
The LiPo battery is going to be used by the heli and for the arduino and some sensors (IMU)...
Thanks!!
Cheers!
Unless you're pulling a lot of current with the sensors there shouldn't be a issue with feeding it slightly above 12V. Its OK to 20V but if you draw a lot of power through the voltage regulator it can overheat if it has to lose too many volts. Mine is quite happy running with the better part of 16V going in the DC socket, providing I'm not drawing a serious amount f current. Put your finger on the voltage regulator when its running, if you can leave it there without pain for 20 seconds, its good to go....... 
Battery capacity in an aircraft is a limited and costly resource, it's really a waste of energy to power a Arduino with a Lipo 3 cell battery as the on board 5vdc regulator will just dissipate the extra energy as heat that could be used for longer flight duration.
Use the +5vdc power from the R/C systems battery eliminator circuit (most R/C speed controllers have this voltage output to power the R/C receiver and servos) and wire it to a hacked and shortened USB cable that will then plug into the Arduino's USB connector.
EDIT: On second thought this doesn't really improve the situation as it just changes which +5vdc regulator dissipates the power. What would be desirable is a R/C battery eliminator that uses a switching regulator. They are available as standalone modules but perhaps some of the newer nicer motor speed controls would utilize an internal switching regulator. You would have to research that.
Lefty