Polls: reliable?

I had been thinking: how reliable are the polls we see on tv, etc? While he is ahead, Obama's numbers consistently remain within the 46%-49% range. He may have more than McCain, but that doesn't mean he has a majority. There is a lot of undecided voters out there who have the capability to sway the election significantly. I believe that the election will result in either a close win for McCain or a landslide for Obama. I hope its the former.