PIN is specific to your BB device, it's a HEX code...that can be used to identify your device as well as send messages to and from your device using BlackBerry Messenger. Loook HERE for more info.
You can find your PIN by going to "SETTINGS" --> "STATUS". The PIN will be listed there. The PIN is basically the ID number for your blackberry. People like myself list their PINs because with the "BlackBerry Messenger" application, you can message other users who are also running the program.
Blackberry Messenger is basically an application that is similar to any other IM application.
Do a search on "BlackBerry Messenger" and you will get all the information you need...
PIN messaging is part of your data package from your carrier, TMobile in your case. PIN messaging won't specifically be mentioned on your bill... only as data used.
Along these lines, there is an autotext entry so that you can type in mypin and hit space and it will replace that with your pin. Also mynumber and myver.
Good Luck!