Depends on the circumstances and even on who is your regulator. Traditional overdraft protection lines of credit have always been considered credit and are subject to full Reg Z disclosures as such. These are often tied to a DDA for repayment, that doesn't change its status. Newer Overdraft Protection Programs don't contain all the same features as the lines of credit and are therefore removed from Reg Z coverage. Typically the element most often removed is the obligation of the bank to advance available funds from the line. With ODP programs, the bank retains the discretion to pay or not pay the overdraft. Even in those programs, however, most of the regulators agree that paying an account into overdraft is a loan (the OTS is the dissenter here, taking the opinion that because these decision lack tradition credit underwriting they are not loans in the truest sense of the word).
As far as reading suggestions, read the recently issued Interagency Guidance on Overdraft Protection Programs and Andy's article on the same -
http://www.bankersonline.com/operations/overdraftreview.html