The traditional rules of English grammar say it should be "whom" because it's functioning as an object pronoun, but many say that that's outdated and it's acceptable to use "who." They say the only time you have to use "whom" is when it comes after a preposition. In your example "who" is object of "about", but it's placed at the beginning of the sentence rather than after the preposition. I think it's fine to us "who" this way in conversation. In fact, it would sound stilted and formal to use "whom." Just remember that in formal writing you should never use "who" as object pronoun.

Whom. Ending a sentences with prepositions often lands you in trouble like this. For a simpler sentence that breaks no rules, write: "Who is the subject of the first sentence?"