I seem to remember seeing somewhere that Thunderclap and Curse of Tongues both overwrite the effect from Deathfrost, so I doubt it would be really useful in that situations.
I believe it works on bosses, but as Metal stated, it is not stackable (from what I have read) so would most likely be a useless enchant, at least for mages.
i consider it to be more of a tank enchant, because slowing attack speed = damage mitigation. the 150 damage are peanuts. maybe it's in anticipation of death knight tanking?
It does work on bosses. I'd imagine it's potentially useful for Tankadins during boss fights in 5-mans, but that's about it; for raids you'd rather have an actual TC effect, and it doesn't stack with it.