Of course 'about' is too casual for a business letter. I'm talking about MN posts in which people saying 'in regards to' or any other variation on that word. It's wrong because it's too formal.
'with regard to' is more commonly used in Brtish English 'in regard to' in American English.
LRD this isn't to do with grammar but collocation. That is, some prepositions go together with some other words for no good reason, just common usage. This usuage can and does change all the time. I'm actually not a pedant because I have no problem with language changing. If lots of people start using 'in regards to' then that is what will be 'correct'; that's how language works.
My main issue is when someone uses a phrase randomly because it sounds more serious, when the situation doesn't demand serious vocabulary.
Anyway, I'll off to pedants corner now. becausei'mworthit can go back to Police Officers' Corner