Its how RIM knows what Blackberry to send your email to. It also allows you to use Blackberry messanger and send "peer to peer" messages to fellow Blackberry owners. To find your pin simply go to options > status. Or just type "mypin" in a message, email, or note,
PIN is the phone identification number, it will allow you to send messages via Blackberry messenger to others with BB. It will also allow quick email messages via PIN. All BB have a unique PIN to them. You can find it by going into the options, status, it is located there
Posted from my CrackBerry at wapforums.crackberry.com
Just to add a little to what sgocka said, each Blackberry has a unique PIN number. Not that it really matters, but it's a hexadecimal number, with digits 0 - F.
When you change BlackBerry devices (and eventually we all do), you'll have a new PIN number for the new device.
