BBM has long been plagued with issues when swapping devices or installing a new OS. For years BlackBerry users have stuggled with keeping their contact list intact and not having to start from scratch after an update. In recent versions, the backup/restore feature has kept us sane (for the most part) giving the option to backup your contact list to an email address or SD card. The wasn't foolproof either unfortunately as many still have issues getting their full contact list back for any number of reasons. Thankfully with BBM version 7 (and going forward) you no longer have to worry as your BBM contact list and settings info is now all tied into your BlackBerry ID.
In BBM 7, your BBID grabs your contacts and settings list and holds onto them. No longer do you have to backup or restore you contacts manully either from your SD card or by email. All the info is stored in with your BBID, so as soon as you login on your BlackBerry, your BBM contacts are populated and you're good to go.
While it may have appeared that this was already the case, it certainly wasn't. If you had backed up your contacts to an email address tied to your BlackBerry, most times your contact list would restore on it's own after a device swap or OS update. Other times, not so much. I can't count how many times I've had to manually add back my contacts one by one after a botched BBM restore.
Hopefully this functionality will run down to other core and third-party apps for BlackBerry 10 as well. One of the longest running gripes of BlackBerry users is having to start fresh from most apps after an OS update. As we've seen on the PlayBook however, that should all be gone when we move to BlackBerry 10 -- so plenty of good things to come!