I found this in another forum. An iPhone user asked the same question and they got this reply. If this is wrong, I apologise.
4 0 "The n letter symbol is from the backgrounder app..."
The N letter symbol is from the backgrounder app. It appears in apps that automatically run in the background. It is there to let you know that you do not need to enable them in backgrounder or something. So you should see it while using phone, mail, and ipod for example.