Here's an issue poll from Nanos, taken between August 28 and September 2. Given a choice, these were the top issues (with change from August results in brackets):
Jobs and/or the Economy - 31.3% (+1.0)
Healthcare - 23.6% (-2.5)
Environment - 8.7% (-0.7)
Education - 8.4% (+4.3)
With the margin of error at 3.1, only education can be said to have definitely become a more important issue for voters. That the poll was taken just as students were returning to school, however, might be the reason for the change.