Benefit: Once per turn, the first target you hit with a power that has the cold keyword gains vulnerable 5 cold after the attack. The vulnerability lasts until the end of your next turn.
Power (At-Will • Cold): Free Action. All damage dealt by this weapon is cold damage. Another free action returns the damage to normal.
Lasting Frost is commonly used with Frost Weapons in the DPR builds over at the wizards CharOp forums. Example here.
However, I'm not convinced that it works due to the emphasized language in the lasting frost quotation above.
Does attacking with a Frost Weapon convey the cold keyword to the weapon powers you use with it?