two recent polls for Iowa, yet there have been many national polls of late. Why? Because it's easier to get a random sample from a larger population, and because most polling companies don't have enough infrastructure to do a proper poll in just one state. There isn't as much demand for it.
So how useful are national polls, especially this far from the date of the first primaries? The answer is that it depends on which candidate's numbers you're reviewing. National polls measure both support and name recognition. Most primary voters begin to pay closer attention to the candidates only a few months before it's time to vote. So if a candidate is unknown, he will not see any support in the national polls.
An unknown candidate may ultimately do well, once he's had the opportunity to introduce himself to the voters of Iowa, New Hampshire, etc. This was certainly true of Mike Huckabee during the last election: He polled near zero percent all year until he gained support in the last few months of 2007, and then he won Iowa. On the other hand, many unknown candidates fizzle out even after the voters discovery them.
National polls, especially early ones, therefore tell you very little about how an unknown candidate will perform. However, If a candidate's main weakness is lack of name recognition (e.g. Daniels or Pawlenty), then seeing him rise in the national polls is a very positive sign.
National polls tell you much more about candidates with high name recognition. For example, if a candidate is very well known but is doing badly in the polls, that's an indication that he will do badly in the primaries. It's not a bad idea to see how well-known candidates stack up against each other in the early polls.
For example: Romney, Huckabee, and Palin all possess a lot of name recognition. If Palin continues to trail behind the other two in the national polls, she is in trouble. If her national numbers go up, she's back in the race.
Still, the state polls of Iowa, New Hampshire, and South Carolina are more important than any national poll.