As of October 1, 2007, the United States Government has begun requiring all travelers outside of the US to have a valid passport. This does NOT include the US Virgin Islands. People still can travel to and from the Puerto Rico and US Virgin Islands with certified birth certificate or proof of application for a passport with a government issued photo ID.
If you wish to visit the British Virgin Islands the above proof is all that is required until the end of the year. As of today, beginning January 1, 2008 passports will be required to enter the British Virgin Islands and to return to the U.S. Virgin Islands.