Formally it should be "about whom". However, conversationally most people would say "That's the man (who/that) we talked / were talking about yesterday".
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lucas21cThen, how about "That's the man about who we talked yesterday?" Is 'about who' very awkward or wrong in the sentence?"who" for "whom" sounds more wrong following a preposition than in many other cases. However, the pattern as a whole, irrespective of "who/whom", seems rather formal (and in this specific case even a bit awkward), so it is not a sentenc