This patch by Teppo Känsälä (teppoka) makes parenting bones with Ctrl-P support the X-Axis-Mirror option for Armatures. It will cause the mirror-bone of the selected bone to also get parented. i.e. if "bone.R" was selected and being parented, "bone.L" would also get parented.
I've made an addition to this patch, which means that if the bone that acts as the parent also has a mirror-bone (i.e. when parenting "lowerArm.L" to "upperArm.L", "lowerArm.R" gets parented to "upperArm.R"). Otherwise, they both get parented to the same parent bone (i.e. a parent bone that doesn't belong on either side such as "upperSpine")