I'd say that one deep reason for Hillary Clinton's slight slide in Iowa is the passion of John Edwards' rehtoric against her. Marc accurately sums up the message:
Hillary Clinton is too emeshed in the corrupt culture of Washington to fix things and to take on the entrenched interests that need taking on. He -- Edwards -- is unafraid.
The truth is that Edwards has been making that case much more cleanly than any of his opponents, whether or not they began to make it first. It's a different case than Barack Obama's -- he argues that Clinton is too polarizing to get the job done -- Edwards takes the charge further: it's Clinton herself who is part of the system that needs reforming, that it's the Clinton legacy that has consigned the Democratic Party to second-tier status.
If Obama wins Iowa, it will be because of Edwards' rhetoric and his own organization. And if Edwards falls below 15 percent in some caucuses, his supporters will surely not put Clinton second.