I took the most recent Scottish independence opinion polls (since August last year), put in the trend lines and extended them until the day of the actual referendum, 18th September 2014. The results can be seen in the graph on the right.
Basically, No is falling quickly, while Yes and Don't Know are rising at a more moderate pace.
The effect is that according to current trends, Yes will overtake No on the 1st of September 2013, and by the time of the referendum, there will be more than twice as many Yes voters as No voters.
Obviously it's very unlikely that the current trends will continue for the next 18 months, and of course opinion never shifts in such a neat, linear fashion. However, it does show that the Yes side is building up momentum, and that it is indeed possible, perhaps even likely, that Scotland will vote yes to independence.