At this point I certainly wouldn't consider Mikko Koivu a bust. In my opinion, people are far, far too liberal with the term bust, often using it to describe players who didn't reach their maximum, usually over-hyped potential. To me a bust is a player who completely flops, either never makes the NHL or only has a handful of games.
It drives me nuts when people call players like the Sedins busts. In that case, they are a pair of players who put up very respectable second line point production last year while playing 12-14 minutes a game only and often playing without a competant winger on their side.
So, is Mikko Koivu going to be a bust? I highly doubt it. He probably is going to be the #1 centre that many heralded him as being either. He probably will never put up 75 or 80 points, but I definitely think he could be a very good second line centre at this point. If I were to guess which way he'd go I'd still lean favourably towards the likelihood of him reaching his potential compared to his 'bust' likelihood.