From NBC's Chuck Todd
Be careful over-interpreting the independents number for McCain in current polls. The reason he's doing well among indies is that a growing slice of them are former Republicans.
This goes to the party I.D. issue. As more folks refuse to identify themselves as GOPers, they move into the independent category, making those voters more conservative than we've seen in the past and therefore artificially increasing McCain's share among them.
When looking at national polls, it's better to study ideology (those who identify as "conservative" or "liberal") rather than party ID ("Republican," "Independent" or "Democrat").
Just something to keep in mind when examining these national polls.