Overdraft Opt-in Rule on ACH Payments

Can anyone guide me to where I can find information on this topic?

No, because there is no such requirement. For FDIC-supervised banks, there is an expectation that banks will offer an opt-out for OD service for ACH and check items, but there's no opt-in requirement.

I am looking at the NSF fee on the new ACH rules for 9/16/2011.

You will find the information in the NACHA Operating Rules & Guidelines book. In the front of the book starting on page OR xxxiii.

The 9/16/11 change involves the ability of a merchant to assess a bounced item fee for ARC, POP and BOC entries, or for qualified checks, when such an entry or check is returned to the merchant for insufficient funds (NSF) or uncollected funds (UCF). There are specific requirements as to notice (or other authorization for the fee). The exact character string "RETURN FEE" will have to appear in the Company Entry Description field. Only one return fee will be permitted for an entry/item, regardless of how many times the entry/item is returned. If the returned entry is an ACH entry that carried a check serial number (ARC, BOC, POP, RCK), that serial number must also appear in the Return Fee entry.

The 2011 edition of the Rules includes this fee under section 2.14 (Return Fee Entries).