In regards to the rules of grammar cited by Topsie et al, is this something new or peculiar to British English as I consulted an English grammar textbook from the 1950s (when I was in school) that was used in North America and they put periods after Mr. Dr. etc. This is the way we were taught over here. Reading today's newspaper I see that they also employ a period in these cases.

Hmmm...this is getting curious, perhaps Peace can provide some insight as he is a teacher (of English?).