If the power uses the Weapon keyword, I imagine you get the weapon's proficiency bonus. Likewise for an Implement (though it appears basic implements do not have proficiency bonuses)
So a power that was Dex vs AC WITH the Weapon keyword would be:
1d20 + 1/2 Level + DEX mod + Weapon Proficiency
while a power WITHOUT the Weapon keyword would be:
1d20 +1/2 level + DEX mod
That seems to make the most sense, and fits with the way the pre-gen characters are designed.
So a power that was Dex vs AC WITH the Weapon keyword would be:
1d20 + 1/2 Level + DEX mod + Weapon Proficiency
while a power WITHOUT the Weapon keyword would be:
1d20 +1/2 level + DEX mod
That seems to make the most sense, and fits with the way the pre-gen characters are designed.