my 2 bits
another comment on this.
a pact dagger would count as a dagger for the rogue using his "weapon" based powers, and rogue training +1 atk melee.
using his one warlock power, the pact dagger would function as an implement, and provide bonuses to atk/dam. BUT the rogue cannot use sneak attack with this attack, as it is an implement attack, not a weapon attack.