Safest bet is to assume the world can read it. I would encrypt the data as that way the transport doesn't matter as much. I wouldn't send any confidential information in plain text unless I was 100% the end to end connection was secure.
Is DropBox itself secure? I would say about as secure as Gmail. There was an issue a while back where passwords weren't being checked or something, and anyone could log into anyone's account. Since then, I only put non-sensitive material there, or ensure it has been encrypted.
Regards,
Mike