You are not purchasing the past due loans solely for the intention of collecting them and therefor you are considered a creditor under the definitions of the FDCPA.
The definition of debt collectors that raises this question is found at 803(6)(F)(iii): (iii) concerns a debt which was not in default at the time it was obtained by such person.
However, the definition of creditor 803(4) indicates: "(4) The term "creditor" means any person who offers or extends credit creating a debt or to whom a debt is owed, but such term does not include any person to the extent that he receives an assignment or transfer of a debt in default solely for the purpose of facilitating collection of such debt for another. "
So basically, the purchase of a portfolio that may have a few past due loans will not subject the bank to FDCPA.
THe FTC commented on this a number of times - see Arbuckle and Trokildson:
http://www.ftc.gov/os/statutes/fdcpa/letters.shtm