Creating a Password and Timeout on a Treo 650/700 (step-by-step instructions)
Go to Home (All Applications Page). Select Security, then assign a Password, then set "Auto Lock Device" to lock after preset delay of no more than 20 min. Click on another function (like email or phone) to exit.

Creating a Password and Timeout on BlackBerry 7xxx (step-by-step instructions)
Go to Options. Select Security, change Password to "enabled", then change "Security Timeout" to no more than 20 min. Hit escape, save your changes and enter in your password. Hit escape to bring you back to the main menu.

Creating a Password and Timeout on BlackBerry 8300/8700/8800 series (step-by-step instructions)
Go to Settings/Options/Security Options/General Settings. Change Password to "enabled", then change "Security Timeout" to no more than 20 min. Hit escape, save your changes and enter in your password. Escape back to the main menu.