Opinion pollers do a lousy job of predicting elections. For a good read, see for example the prediction of the New Hampshere primary in 2008, when all polls predicted Obama to win, but it was Clinton who won (albeit by a slim margin).
In the Dutch context, there are three main polling firms, that each do equally well (or badly). Out of a hundred and fifty parliamentary seats, peil.nl mispredicted 20, while TNS-NIPO and Synovate shared the honor of only missing the target by 16 seats in the 2010 parliamentary election. These polls were conducted the day before the election, and some of the pollers said that people might have changed their vote at the last minute. That may very well be, but even the exit poll on the night of the election was wrong. Peil.nl was 17 seats off and TNS NIPO 15. Only Synovate did a lot better, and only missed the true result by 3 seats. I will discuss why this is in a next post, but it is just a matter of speed and low costs versus quality.
And we have known for a long, long time how to do exit polls. Although there was public outcry in the UK, when the exit poll predicted the liberal democrats not to win the elections, it was spot on. If we know how to do it, then why don't we?