A "lo-tech" suggestion -- Ask the customer to provide a voided deposit ticket or check on each account they want to access. At least this will give you some assurance the customer is accessing his/her own accounts.
Be aware that giving the customer ACH access -- even if you control where the funds can go -- makes your monitoring of transactions for suspicious activity the more complicated.
[This message has been edited by John Burnett (edited 01-09-2002).]