Honestly, I hope it's not the serial number of the iPhone. The one thing I hated about BBM was whenever I changed to a new BB, I had to announce to all my contacts on BBM of my new PIN. Very annoying and some users are too busy to immediately add.
To make it future proof or less hassle for the end user (ie upgrading iPhones every year or two), the PIN should be the ICCID which identifies the GSM SIM card as the SIM card goes with you...unless you switch carriers. You can get the ICCID in Settings > General > About.
Would be added bonus if you have a feature in your app to pull/copy the ICCID to share it out to other iPM users to add to their list.