Very few (if any) fictional products are actually registered trademarks, however they are creative works. This means any use of similar packaging is subject to copyright infringement laws. There is a possible case against a product using only the same name, if it's reasoned the name could cause confusion.
The best example to understand this situation would be Duff Beer from The Simpsons. That wiki page lists several instances of beers named Duff around the world. Some have had legal action sought against them by Fox (names only as well as name and design), and some have escaped any legal action. Interestingly, a man in Mexico has managed to register 'Duff' as a trademark in Mexico.