If your blackberry cannot handle 72 characters that's a mail reader limitation most people don't have.
Would you rather have someone email you with:
1) Subj: Can you print an extra copy of the Excel for 10AM? Thanks! [EOM]
2) Subj: Quick request for you before our meeting today Body: Can you print an extra copy of the Excel for 10AM?
Personally, I much prefer the first.
Do most people know what [EOM] means? I'd never seen it before. Given the context I guessed it means "end of message" or "entirety of message", but were I to have first seen it as part of an E-mail I'd have no idea and would just go read the body.
One should always write for their audience, not some given set of rules.
No, I don't think most people have seen it before, but they seem to figure it out and several of my clients have adopted the habit. No one has been annoyed, or confused.
Getting over my fear of people reading too much into my email tone has been a huge productivity boost in general.
It probably stems from the fact that I visually filter by email address more than subject line.