The verses in question are the following (taken from Ven. Thanissaro's translation):
As a mother would risk her life
to protect her child, her only child,
even so should one cultivate a limitless heart
with regard to all beings.
In short, Ven. Thanissaro claims that the passage that makes the analogy of a mother risking her life to protect that of her child is really referring to the way in which we should protect and cultivate our own attitude of metta not (as I have always assumed) the attitude to be taken towards all beings. Anyway, I would be interested to hear if anyone has encountered this reading anywhere else and general thoughts on the matter. Metta.