GM Service Manual Online
For 1990-2009 cars only

The lockout protection feature may prevent you from locking your key in your vehicle. Pressing the power door lock switch will lock all doors, then unlock the driver's door, if the key is in the ignition when a door is opened.

If you leave the key in your vehicle but not in the ignition or if you use the manual door lock, you could still lock the key in the vehicle. Always remember to take the key with you.