Apparently several plans have been advanced with different schedules for implementation. When several foreign government agencies complained that this plan will not allow enough transition time, the following timeline was put out to allow some wiggle room. The new requirement will occur in two phases: Jan 23, 2007 if traveling by plane, and as early as Jan 1, 2008 if entry is by land or sea.
This will allow ample advance notice to obtain passports or other documents needed for land/sea entries. For air travel, the deadline is here.