This is part of the reason I belive in having some type of hardened entry or foyer. A locked storm door would allow you to open your front door but not allow a forced entry while the door is being opened. >>> Real security doors open out with pins at the hinges to prevent taking the door off. I have a steel one with 3/8 inch plexiglass. Battering ram will do nothing to the actual door.
There are door reinforcement kits. Door Devil, and StrikeMaster II. Under $100.
It does help to have a vestibule...and a security system. I live in a very low-crime area but I have cameras all over the place.
Also helps to live in a gun-friendly state. :)