This term is used by BlackBerry users when someone asks then for their PIN and they do not want to give it. The term can also be used in a sarcastic/comedic way (see example 1b below).
The term rose to prominence through the backlash to instant messaging in favor of more traditional terms of communication.
a) Girl: Hey Charlie what's the deal? You never send your PIN?
Charlie: Are you for real? Don't Tell Me BBM/DTMBBM!
b) Girl sees a hot guy: Mmm, he can tell me BBM any day of the week.
c) Guy sees unattractive girl: Jeez, DTMBBM if you know what I mean.