What regulation states that you can not place an actual account number on an ATM receipt??

None that I know of (unless your State law addresses it), but it would be a very - very dangerous practice.

Agree with rlcarey, there is no federal prohibition as such. I am certainly old enough to remember many changes in compliance requirements, but have not always worked with Regulation E. Isn't it true that, at one time, the account number was required to be on the ATM receipt. However, since people just threw them on the ground, that led to identity theft and they amended the regulation to eliminate the requirement?

Currentlly, the only time an account needs to be identified is when the consumer has multiple accounts of the same type. From the commentary to 205.9:

3. Access to multiple accounts. If the consumer can use an access device to make transfers to or from different accounts of the same type, the terminal receipt must specify which account was accessed, such as ``withdrawal from checking I'' or ``withdrawal from checking II.'' If only one account besides the primary checking account can be debited, the receipt can identify the account as ``withdrawal from other account.''.

Actually, there has never in my memory been a requirement that the actual account number be on the receipt.

It is a Visa requirement. From the Op Regs: Account Number: (Effective through June 30, 2006, except for the final four digits, the Account Number on the Cardholder’s copy of the Transaction Receipt may be disguised or suppressed. Effective for Electronic Transaction Receipts generated from
Point-Of-Transaction Terminals installed on or after July 1, 2003, except for the final four digits, the Account Number on the Cardholder’s copy of the Transaction Receipt must be disguised or suppressed. Effective July 1, 2006, except for the final four digits, the Account Number on the Cardholder’s copy of the Transaction Receipt must be disguised or suppressed.)
So if the transaction routes through Visa, your receipts can only have the last 4 digits visible.

The Visa rule may be referring to the cardholder account number, which is not necessarily the deposit account number. It's not unusual for an ATM receipt to truncate or mask part of the card number, but print the entire deposit account number (if at all).

12 CFR 205.9(a)(4) says the receipt needs to identify the customer's account or access device, but the number need not exceed 4 digits.