When I was in high school (class of 1981), no one went "to prom" or took someone "to prom". It was always "the prom." You don't take someone to movies, or to restaurant, or to school dance, or even go to senior breakfast.
But in the last two or three years, that's what I'm hearing. Oh, people still say "the prom" as part of the prepositional phrase, but I'm hearing the phrase without the article a lot lately - I'm thinking about last night's Without a Trace, but also Grey's Anatomy last year. Or was it two years ago? Whatever - the one with the prom in the hospital.
When did people start dropping the "the" when it comes to the prom?
Okay. From what I've gathered, it's mostly a regionalism that may have been made more general by the movie Pretty in Pink.