I doubt it. He had a solid rookie year but there can only be so many "next Montanas." There are already at least two QBs in the league that are already called that (Manning, Brady, and occasionally Favre but he had a bit of a fallout in the media.) They are years older than Flacco though, so he'll have room to shine once they step out of the limelight.
Joe Montana is defined not only by his regular season success(excellent stats and win/loss percentage), but his great post season numbers(especially Super Bowl). I do not think anyone will ever match his dominance in the Super Bowl. Flacco looks impressive. The Ravens defense got weaker but their offense is much improved. I hope Flacco keeps it up.
Flacco has immense talent and I think the Ravens have something special. This year he will turn the ball over a lot, but that is because he is still learning and the Ravens have just started to open up the offense. He is also going to make some big plays, and I think next year as long as he stays healthy, Flacco will be a Pro Bowler.
Even if Flacco turns out to be comparable to Joe Montana you still can't call what he's done in one year enough to touch a legend such as Joe Montana. Give him time and we'll see where he goes. I have no doubts that he'll be running that offense for years to come though.