This business of a backpack making you unprofessional is ridiculous. As long as it is in decent condition, I don't see anything wrong with it.
Often times in these discussions, backpack opponents ask if one would bring a backpack to an interview. My answer is no. However, I wouldn't bring my aged *brain bag to an interview either. BTW; I don't carry a backpack for work. I do see how some would prefer it to lug around their things as it can be a practical tool.
As for MP3 players (why iPods get singled out, I have no idea), personally, I wouldn't listen to music while walking in a terminal, uniform or not. With electric carts, crowds, people asking for directions, etc. It is just impractical in that environment. However, if I'm sitting at a gate and relaxing, uniform or not, I don't hesitate to listen to an MP3 player and I am every bit as professional as one that isn't using one.
Also, how many of your draw a distinction between a uniformed pilot with wired earbud for their phone or a Bluetooth ear piece versus an MP3 player? With regard to appearance (for backpacks and mp3 players, appearance is what we're talking about, right?), there is effectively zero difference.
*I do find it unprofessional for a uniformed pilot to advertise their political affiliations on their luggage. I wouldn't go to a doctor if they wore their politics on their armband.