The USVI is absolutely part of the United States, and they are US citizens. Now, it's not as straightforward as Puerto Rico, but generally, a drivers license is all you need. It's similar to traveling to Mexico or Canada prior to Fortress Amerika Days, when a drivers license got you through 95% of the time with no hassle and for the 5% of the times you got guff from Immigration, a voter ID card or something along the lines stops them cold.

Mike, as for asking for a birth certificate for the baby, do you think that all domestic travelers should be forced to carry them when traveling with kids? Kids are kidnapped all the time and just about anywhere, right? What "baby trafficking" is coming out of St. Croix anyways?