cpzinga:
My status changed to the following "We have finished processing your passport, and it has been mailed to you. Your passport application locator number is..........You requested delivery by regular mail. Passport Agencies use Priority Mail. This means you should receive your passport on or about 10/20/2009. Check the USPS status at................."
I got an email on 10/17 with the above status since I had also provided my email address. I then checked the USPS online tracking and it had already reached my post office early that morning. So out of curiosity I thought let me check my mailbox and "Wow!" it was in there and a separate envelope also had my nat. certificate. The exact same for my wife (passport and nat cert). So in affect we received it the same day my online status changed (3 days before the delivery date they had suggested in the email).
Also SunnyDays, I too had left the travel dates blank; so that is probably not causing your delay. Maybe as dp4k suggested it could be based on the processing center. I somehow thought that all passports are processed centrally except for the cases where one takes an appointment and goes personally to the processing center but I could be wrong.
Good luck to all of you waiting.