The first thing to remember is that not all polls are created equal, or, more accurately stated, their methodologies are vary widely. Some polls, for example, use recorded messages via auto-dial, which means they can't call cellphones. These polls have to make a lot more phone calls to get a decent sample size, since most people hang up as soon as they hear a recorded message.
Some polls are conducted online. Some polls use real live people making real live phone calls. So how do you know which polls to trust? You don't. That's why smart commenters and armchair pundits use a polling aggregator, a website that polls the polls, adjusts for variances of sample size and historic tendencies, and publishes an average of all the polls.
There are number of good sites to check out. My favorite is fivethirtyeight.com, Nate Silver's nerderiffic blog that offers more polls and poll analyses than you can shake a cellphone at. Silver was on the money with the electoral results in 2008 as an independent blogger. The New York Times then hired him to do the same schtick for its website. Highly recommended info on not just the results, but also the methodology of polling.
Another good aggregator site is RealClearPolitics. RCP is seen by many as leaning GOP, but that, IMO, is primarily because they call lots of states "tossups" that other aggregator sites put in one or the other candidates' column. Silver, for instance, only has seven states that he considers still to be tossups. RCP has 11 such states, including several deemed by most polls to be in Obama's column: Michigan, Wisconsin, Pennsylvania, to name three. However, in 2008, RCP came the closest of any polling organization to the actual national popular vote percentages for McCain and Obama, so I don't discount them.
One thing almost all aggregator sites will make obvious by their comparisons is that Gallup's national popular vote survey is an outlier — way out of line with almost every other poll's results. That said, putting any stock in any national popular vote poll is a sucker's bet. The only numbers that matter are the Electoral College totals. And at this point, the only states that matter are the tossups, however you define them. Most aggregators have Obama ahead in the majority of swing states, and one or two points behind in the national popular vote. Which is much better for him than the other way around.
If you look hard enough, you can eventually find an aggregator that will make you feel good about your candidate's chances. For example, when I'm feeling a little discouraged, I just go here. I call it my "happy place." It'll have to do until November 7th.