well how did ha recenter the pivot? O_o sound kinda funny.. i mean it should work if you do it like you said o_O

but theres always the other way of mirroring stuff..
take your un splitted head, center the pivot, select half the head faces and hit delete..
then go to edit > duplicate > options ans set the 'side' axis (usually x but that depends on witch direction youve modeled it in, try and see) to -1 this will create a mirrored version of your bunny head..

you may need to reverse the normals on it(bcuz you scaled it 'inwards' the normals will be pointing to the inside of the head) but im not sure

but the subdivs > mirror should work too O_o check again where your pivot is.. but as you described it it SHOULD work
i tried it with a sphere and it worked for me O_o maybe deleteing history whould help..