As a follow up, the earliest use of the term that I was able to find is from the 2nd report from the US Commission on National Security/21 Century, which was published on April 15, 2000. While this is the first use of "homeland security" I could find, it should be noted that the first phase report, issued in September 1999, frequently refers to "homeland defense" in a consistent context. Now, while the term "homeland defense" has been used for quite some time, historically when referring to the Soviet nuclear threat, the first time I have been able to find it used in our current context is from a Council on Foreign Relations (CFR) Roundtable titled "Terrorism and Homeland Defense" from April 12, 1999. Here is a link
For those of you who are wondering what the CFR has to do with this, I point you to a previous post of mine, here (#52 - second one down). Also, I have found poking around the CFR webpage to be helpful when considering how we got to where we are today in terms of "homeland security".