Well, Allan, there is a passing reference to truncation in Reg E, section 205.9(a)(4):
(4) Identification. A number or code that identifies the consumer's account or accounts, or the access device used to initiate the transfer. The number or code need not exceed four digits or letters to comply with the requirements of this paragraph (a)(4).
The emphasized text was added to this paragraph at about the same time Visa and MasterCard were making noises about mandatory truncation.
A lot of ATM receipts include both a card number and an account number. Ideally, both would be truncated.
Another area in which some people are suggesting number truncation is the account number on periodic statements. Regulation E doesn't allow for truncation in its periodic statement requirements. It simply says that the account number must appear.