Are there any cases of domestic (u.S.) dengue fever?

Yes. It occurs occasionally in the U.S. - especially south florida and puerto rico. You can follow current surveillance about dengue from the cdc. http://www.cdc.gov/dengue/.
Travel/ immigrants. Most cases in the continental states are related to world travel and immigration. According to the centers for disease control and prevention, U.S. Citizens are impacted in endemic areas including puerto rico, guam, samoa and us. Virgin islands.