If you do some quick math based on the Iowa entrance poll, you see Ron Paul and Mitt Romney bunched in the low 20s and Santorum slightly behind them at 19 or 20. But there are reasons to think the entrance poll understates Santorum's support. That's because a caucus is a much more public act and than the typical balloting exercise. People vote in front of their friends and neighbors, not in the privacy of a voting booth. That arrangement would seem to favor a candidate with momentum, like Santorum. If you showed up intending to vote for Bachmann, Gingrich, or Perry, only to find your fellow social conservatives or Tea Partiers backing Santorum, you might very well do the same. And, of course, that last minute change of heart would go undetected in the entrance poll.
Update: This was true as of the last entrance poll. It looks like the entrance poll is now being updated with actual returns, so it's getting more accurate.