The mainstream media is having a field day with the news that a Zogby poll shows McCain now ahead of Obama. Of course, maybe this is just because I've been reading the Wall Street Journal lately.
Polling is interesting, in that polls done for the media really don't need to be very accurate, and in fact it is better for the media purchaser paying for the poll if they aren't very accurate. The "McCain ahead" numbers are getting such interest precisely because they are new information; otherwise they would just be one more poll showing Obama with a slight lead.
Pollster.com shows the Zogby poll of 1089 likely voters conducted August 14-16 with McCain ahead 46-41, but the Zogby poll of 3339 voters conducted August 12-14 with Obama ahead 44 to 42. Did you hear much publicity about THAT poll? No, because it wasn't "news". Other polls done at the same time continue to show Obama with a slight lead.
Andrew Gelman has some interesting charts showing that you can't tell much from polls done a few months before the election (remember president Dukakis?)
By the way, don't get me started on the whole "margin of error" business. What a joke that is!