In addition - but very important, the geography of a Presidential poll is very significant. Everybody knows the District of Columbia and Massachusetts will vote overwhelmingly Democratic, Alaska and several small Mountain and North Central States will vote Republican (if not necessarily as overwhelmingly). Thus, polling of swing States is much more meaningful.
Polls are not cheap. I wonder how some small newspapers have the money for a poll, especially because, with few exceptions, newspaper circulation is down. Even THE NEW YORK TIMES has laid off hundreds of employees. THE CHRISTIAN SCIENCE MONITOR is folding. Some people have suggested that the Obama campaign is paying for some polls, to keep up momentum. I have no evidence that this has occurred. However, many lesser media outlets are unable to pay for polls. Thus, it appears that some polling is used to drive results rather than to provide a snapshot in time as to a candidate's support.
Cliché that it is, it's worth repeating: There is only one poll which counts - election day. In those States which have early voting Democrats and Republicans appear to be neck and neck. We well may be in for a long night on November 4.