Sure, it's volatile. But do you think Justin Bieber chose BlackBerry because he wants one, or because he got a huge paycheck to use it? Product placement does not end at television and movies.
You might be right about the product placement, but a BlackBerry is still a sensible choice for Mr. Bieber's line of work: touring the world, constantly meeting with people, etc.
The Justin Bieber crowd will pick it because it's used by him, not because they particularly care about the product itself. Obviously not all his fans would value his choice over their own, but there will be many.
And sure, they're volatile, but I'm not saying that Justin Bieber is the reason their consumer-friendly approach wasn't neccesarily a bad idea, just that it currently does, and will in the future, produce more celebrity (non-verbal) endorsements (by using them).
Honestly, look at a blackberry bold (which is a $500 phone, wtf?) then look at an HTC evo (the same price). You would have to be absolutely out of your mind to get a blackberry unless you were using BES. I doubt that the justin bieber crowd is using BES.