Actually, just read something interesting. This is a USAToday/Gallup poll, and it actually differs from Gallups own tracking polls. However, there is a +/- 4 margin for error, they had Obama ahead 48-40, the next poll showed McCain ahead 49-45. Accounting for the margin of error, in both cases they could be in a statistical tie, and the rather large (12 point) swing could be due to a sampling error.