Spells almost always use implements rather than weapons.
A weapon used as an implement ignores the weapon proficiency rules.
Basically, there's no advantage to having a non-magical, non-special implement when casting a spell; only those with magical or special properties give you any kind of advantage (bonus to attack & damage, etc).
Attacks with the Weapon keyword that are made with a proficient weapon get the weapon's proficiency bonus to attack as well (usually +2 or +3). But again, Implement powers ignore that bonus even when the character is using a weapon as an implement. This is because Weapon powers typically target AC while Implement powers nearly always target NADs, which are (again, as a rule) about 2 points lower than AC.
EDIT: As for references, sorry, IDHMBIFOM.