Take a look at the tensioner to make sure there aren't any obvious signs of damage to it. Put a ratchet on it and and test to make sure that the tensioner spring is still putting out an adequate amount of force. The spring could be broken which would explain the problem.
Also, stand on one side of the motor and sight down the face of the pulleys to check for a bent pulley or accessory shaft. This is easiest done while the motor is actually running as the tell-tale wobble will be much more obvious. Of course, you can always run a straight edge across the faces too but it's absolutely critical that the edge is absolutely straight.
Unless something catastrophic has happened recently though, I doubt a bent pulley is the case. Pulleys and shafts don't bend on their own. Of course, if you've had the belt overly tight, that could do it.
on edit:
Another possible cause is one of the accesories being loose in it's mount. Double check them too.
