I don't put too much faith in the poll—if for no other reason than that national polls really don't mean anything.
As the last two elections should have made clear, the presidential race is not a national race. It's a collection of state races. And even then, only a handful of states really matter. Winning the "national" vote won't get you a cup of coffee—just ask Al Gore.
Yet while national polls may not mean much as a direct barometer of who's ahead in a presidential race, they can serve to energize (or deflate) a candidate's supporters—which in turn contributes to the degree to which they'll get involved. And in this respect, Zogby's poll is an obvious benefit to McCain. Tim Carney sums it up nicely for the Evans-Novak Political Report (item 3):
A Zogby poll showing McCain ahead by five points should not be taken to indicate he is the front-runner, but it certainly helps puncture the notion that Obama is way ahead.