For total junkies who want to know everything about how this Iowa process works, go generally to www.caucus2004.org. There are some great one-page bullet sheets about, for example, how the 15% rule actually apportions the more than 13,000 delegates that will be sent from the 1,993 precinct caucuses tonight to the next, county-level round of the process that occurs in March.
And if you really have way too much time on your hands, you can peruse this official, comprehensive manual on everything about how the caucus works (link is a PDF).