The rules compendium directly addresses this issue:
Rules Compendium p.275:
Weapons as Implements and Vice Versa
Some adventurers have the ability to use a weapon as an implement or an implement as a weapon, and some magic items grant this ability. For example, the holy avenger weapon can be used as a holy symbol, which is a type of implement. The following rules govern such uses.
Using a Weapon as an Implement: If an adventurer is able to use a weapon as an implement, the weapon works like a normal implement, but the adventurer uses neither the weapon’s proficiency bonus nor its nonmagical weapon properties with his or her implement powers. For example, if a weapon has the high crit property, that property does not work with implement powers if the weapon can be used as an implement.
When an adventurer uses a magic version of the weapon as an implement, he or she can use the magic weapon’s enhancement bonus, critical hit effects, properties, and powers. However, some magic weapons have properties and powers that work only with weapon powers. Also, a weapon’s range and damage dice are usually irrelevant to an implement power, since such a power has its own range and damage expression.
Thus the +2 Songbow would grant you +2 enhancement to your implement powers and +2d6 damage on a crit, just as it would for weapon attacks, but not the proficiency bonus.