While it is a possibility I have always been suspicious of polls (that isn't to say they can't be correct) as they are simply snapshots of the time and location they were taken in.
At a guess I would imagine a poll in Miami would vary through the day as you have a large community of retirees who are available to answer the phone through out the day where as an evening poll would increase the number of folks home from work who have a different set of priorities when choosing a leader.
Once again I tend to see polls as a general trend indicator but not necessarily an accurate one.
One thing I did find probably more interesting than Powells endorsement was that of Charles Fried...
Reagan Appointee and (Recent) McCain Adviser Charles Fried Supports Obama
Charles Fried, a professor at Harvard Law School, has long been one of the most important conservative thinkers in the United States. Under President Reagan, he served, with great distinction, as Solicitor General of the United States. Since then, he has been prominently associated with several Republican leaders and candidates, most recently John McCain, for whom he expressed his enthusiastic support in January.
This week, Fried announced that he has voted for Obama-Biden by absentee ballot. In his letter to Trevor Potter, the General Counsel to the McCain-Palin campaign, he asked that his name be removed from the several campaign-related committees on which he serves. In that letter, he said that chief among the reasons for his decision "is the choice of Sarah Palin at a time of deep national crisis."
Fried is exceptionally thoughtful and principled; his vote for Obama is especially noteworthy.
It must start hurting your campaign when your own advisers start voting for the opposition.