[sblock=build thoughts]Cold Focus: the point of Lasting Frost/Wintertouched (which I was going to go for when I have the feats) is to get CA all the time. As such, you want at least one cold-based At-will power to spam it with.
Sorcerers... well, I don't know if there's a frost dagger the way there's a flaming dagger. As a property I think you can make your damage cold damage, so that should be fine. Good implement, that.
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Frost weapon is in the PHB
PHB said:Power (At-Will ; Cold): Free Action. All damage dealt by this weapon is cold damage. Another free action returns the damage to normal.
Now, the question would be; does the weapon deal damage, if used as an implement? I believe somewhere it was said the wintertouched/lasting frost combination worked for melee but not spellcasters, since the implement is not actually dealing the damage. Which doesn't make a whole lot of sense to me, but that's what I've heard.