While you can't currently get the position of a mob (I'm surprised noone ran this test when this was possible :), you can still get the current player position. I used this in conjunction with the graphical Blizzard/Flamestrike trigger (with a known range of 30 yards) to determine at what distance would a mob engage my character. Using a little custom LUA script (with the MapLibrary), I would position my 38 mage at the maximum cursor range for Flamestrike/Blizzard from a standing mob, store my location and slowly move in a straight line towards the mob until it aggroes me, when the +combat event triggers the calculation of the distance from the first point. The first test showed a rate of 0.9 yards/level difference, however it seems that the graphical cursor range of Flamestrike and Blizzard is only around 27 yards (tested, considering Blink range as reference, which was correctly jumping 20 yards), which brought me to the 1/1 raport.
Asymmetrical aggro radius
So why don't we find some mobs that are standing completely still and repeat the test?