Distance between 2 arduino's using rf links
That's ingenious, but unfortunately it is liable to be a very inaccurate system with poor repeatability. Range will be affected by antenna characteristics, and alignment. Power level would vary between units. Local conditions and RF noise will have some effect. 

Other options include  


Acoustic, usually ultrasonic - time of flight or triangulation. Sometimes amplitude as you are doing.
RF - time of flight or swept Doppler Radar or triangulation. 
LASER. Similar to RF, Phase detection, ...


Many of the robot sites discuss range finding methods.