There are holsters/pouches that come with a magnet installed in them.
When your BlackBerry hits the magnet, it will enter somewhat of a sleep mode.
Turns the display off, turns the profile to the In-Holster option, so that maybe it will vibrate, or whatever.
When you pull the BlackBerry out of the holster/pouch, Display will return, and your profile will as well.
My BB is on Loud most of the time...
When it's Out-Of-Holster it rings,
When it's In-Holster it Vibes, and for certain things, will also ring.