From D&DIC:
Level: 30
Price: 3125000 gp
Weapon: Axe, Hammer, Heavy Blade
Enhancement: +6 Attack rolls and damage rolls
Critical: +6d6 radiant damage, and you can spend a healing surge
Property: A holy avenger deals an extra 1d10 radiant damage when the power you use to make the attack has the radiant keyword.
Power (Daily): Minor Action. You and each ally within 10 squares of you gain a +5 power bonus to Fortitude, Reflex, and Will defenses until the end of your next turn.
Special: A holy avenger can be used as a holy symbol. It adds its enhancement bonus to attack and damage rolls and the extra damage granted by its property (if applicable) when used in this manner. You do not gain your weapon proficiency bonus on an attack when using a holy avenger as an implement.
Yeah, clerics can use a Holy Avenger Weapon as an Implement.
As it stands here are the list of implements, as well as the classes that can use that implement (not counting Spiral Wizards' feature):
Holy Symbols and Holy Avenger weapons- Clerics, Paladins
Blades- Swordmages
Pact Blade Weapons- Warlocks (by the item property), Swordmages (by the light blade requirement)
Orbs- Wizards, Artificers
Rods- Warlocks, Artificers
Staves- Wizards, Artificers
Wands- Wizards, Warlocks, Artificers
At any point where you have two classes for the same implement, that implement can be used for the Implement powers of both classes by the same character, IF you are given that ability by the class.
Half-Elves, for example, do not get to use your Implement enhancement bonus for your Dilettante ability unless you also get the Multiclass feat that gives you the right to use those implements.