Yeah, that's basically what I was trying to say. You can't flip a switch in the phone itself to turn on holster mode. It's always ready.
The description on the BlackBerry site says:
Plus, a magnetic proximity sensor determines if your BlackBerry smartphone is in its case and shuts off the screen to conserve battery life, while giving you the option to choose unique notification settings for incoming calls and other functions.
All of the resellers seem to have copied that directly, so I assume that the official case will be able to trigger holster mode.