I've now heard this a handful of times second-hand, but from people I trust: Hillary's internal tracking polls show her ticking up in the last week or so. The rationale I hear most often is the Des Moines Register
poll endorsement, which affirmed her experience, preparedness, etc.
One other thing worth passing on. You hear a lot about how high turnout would benefit Clinton and Obama, since their targeting and turnout operations are the more sophisticated and aggressive. (See this piece in today's New York Times, for example.) But a caucus veteran I spoke to today adds an important point: Even if no more people caucus this year than the 120,000-125,000 who turned out in 2004, probably a quarter to a third of those people will still be new caucusgoers. That's just a function of natural churn--people moving into/out of the state, dying/becoming eligible to vote, moving into/out of jobs that force them to work nights, etc. Which is to say, even if turnout stays flat, Clinton and Obama could both benefit from their mechanical advantages.
Update: Just to clarify, just because Hillary's internal polls show her ticking up doesn't necessarily mean they show her leading. I honestly don't know what they show in absolute terms; all I've heard is the direction of the movement. I stipulate this because I just caught a small earful about this from former Iowa Democratic Party chairman Gordon Fischer--an Obama supporter--who was pretty insistent that the race is a dead heat. That may well be true. In fact, it would be consistent with the numbers I alluded to and the CW from a few weeks back that Hillary was a little behind.