I don't see massive damage as a death effect.
That said: what really matters is the condition of the body. Massive damage represents a blow powerful enough to kill you WITHOUT chewing through your hitpoints. Nobody is going to survive if you whack their head clean off, no matter how many HD they have. Unlike Resurrection or even Raise Dead, Breath of Life does nothing to change the condition of the body aside from granting it more hit points. Yeah, you could probably Breath of Life someone who died due to massive damage, but they might be brought back sans-head, or cut in half from shoulder to waist, or missing their arm at the shoulder, or torn neatly in half by a dragon bite.
That is an interesting point of view Nails. Does this mean that cure spells don't fix broken bones, punctured lungs, or open wounds? Because I would say they do. Any of the physical damage which would prevent the body from maintaining like would be resolved. Or else cure magic would be mostly useless. I mean if someone breaks their arm you can restore their HP but their arm is still broken? Sorry that doesn't make sense to me.
The headless effect is actually vorpal which I believe is more like a death effect...the weapon enchant actually uses circle of death to make...so yeah I could see it not working on vorpal but massive damage and vorpal aren't the same.
RAW as written breath of life would work for massive damage...obviously it is the DM's call.