Did you get a holster with your 8100? It needs to be in a holster with the magnet in it which puts the BB to "sleep". It works via the magnet telling the 8100 "You are safe now, baby, go to sleep... all is well." lol, sorta.
As an aside, if you want a horizontally oriented holster, look for ones designed for the RAZR. They often have magnetic flaps and the one T-Mobile sells is the perfect size and works fantastically with the BlackBerry.