Because I'm a Bad Person, I no longer remember who pointed me to Halfway There's primer on polling, but it's really an excellent of the effects of sample size, and why it's legitimate to project results based on small numbers of interviews. Some important notes from the conclusion:
Second, even a poll that is supposed to be within its estimated margin of error 95% of the time will be wrong and fall outside those bounds 5% of the time. That's one time in twenty. Therefore, whenever you see a political poll whose results seem way out of whack, it could be one of those flukes. Remember, polling is based on probability and statistics: it's accurate in the long run rather than in every specific instance. In a hot contest where lots of polls are taken, a candidate's campaign is likely to release only those polls that show the candidate in good shape. The 5% fluke factor may be just enough to keep hope alive among those people who believe everything they read.
Pollsters take their results with a grain of salt, so you should, too. But it's not because of sample size.
Read, as they say, the whole thing.
You may have seen my polling article cited by Coturnix. He linked to it over at A Blog Around the Clock. It's since been linked to by Crooks & Liars, whose name clearly demonstrates that it's a blog about politics.
Someone else here also linked to it later. Perhaps Mike the Mad Biologist in one of his linkfests?