I have read that both Rio Ferdinand and Daniel van Buyten began their careers as strikers (I have even managed to train van Buyten to be a top striker in Football Manager). Their careers as strikers have been unremarkable though. Is there any player in history to have a fair amount of success as both striker and defender (someone who has been featured at least once in the World Cup)?
If one just takes your question by its title then obviously there are plenty of strikers who also comply significant defensive roles. In fact in the famous game Football Manager there is a specifically designed role called "Defensive Forward" who closes down the opponent a lot and try to tackle. In real life the most famous example of such a forward is Wayne Rooney, who besides his attacking talents also retrace back very deep into his own half and tackle opponents very well.
If you want to ask "a player who has played both as a striker and as a defender" then such an example is probably much harder to find, especially at top European football. One close example would be Fernando Hierro who had a remarkable score rate even though he played as a defender. He once even scored a hattrick in La Liga and he's the 4th highest score for Spanish national team. For a not-so-famous player, the Chinese player Fan Zhiyi who won Asian Footballer of the Year plays as both central forward and defender. Well actually he participated in 2002 World Cup so that fulfills your requirement here.
There are some instances of defenders playing as strikers. One motivation for putting a defender up front is if the manager felt that they lacked size in the box. In the 2012 euros (Or was it another tournament?) Del Bosque substituted Javi Martinez (a CB/DM) as striker while Fernando Torres stayed on the bench.
Another case I can think of was Jürgen Klopp, who recently managed Borussia Dortmund, who played as a striker/defender.
On the other hand, there isn't quite as much motivation for putting a striker as a defender. That being said, strikers play an important role for the out-of-possession team. A good striker while press the defense and try to force errors, or try to force the defense to play out the ball to a certain side.
Adriano (brazilian) who played in Barcelona FC and brazilian national team was used as defender and as striker by Joseph Guardiola (though not many times in the last role). He scored several goals as a defender and he goes up to assist position several times running through the left or right side of the field so he was a good choice. Guardiola in Barcelona made several of these role changing, he wanted their players to be "multi-role" though I dont remember if there was another defender who was used as striker, Dani Alves is probably another case. Recently with Luis Enrique coach, Sergi Roberto in Barcelona FC was used as defender, midfielder, and striker. He was selected for the spain national team already. In the 70's "Mechanical Orange" from Netherlands, there probably were some, because the idea of highly multi-purpose players comes from that team and was later incorporated by FC Barcelona when the leader of the mechanical orange Johan Cruyff went to play and later be coach in that club