We usually have some granola bars in our bags when we go to Disney parks and there's never been a problem.
You'd probably have a harder time "sneaking" in a couple of Subway subs, though....
Actually...no, I brought in a sub for lunch one day. Guard made a joke and said "This looks delicious! My lunch break's at 11:30...where can I meet you?"
Disney can't enforce a rule against food and drinks because some people will go without due to cost or stubbornness.
The first person to die from low blood sugar or dehydration because Disney wouldn't allow them to bring in their own food would be a legal nightmare.
This is from the website:
Q. Are there any personal items I should not bring with me into the Theme Parks?
A. Items that you may not bring into the Theme Parks include, but are not limited to:
* Items with wheels, such as wagons, skateboards, scooters, inline skates, shoes with built-in wheels, two-wheeled or three-wheeled conveyances, strollers larger than 36" x 52", suitcases, coolers, or backpacks with or without wheels larger than 24" long x 15" wide x 18" high (coolers required for medication may be stored in a locker or at Guest Relations), and any trailer-like object that is pushed or towed by an
ECV wheelchair or stroller
* Alcoholic beverages
* Weapons of any kind
* Folding chairs
* Glass containers (excluding baby food jars and perfume bottles)
* Pets (unless they are service animals)
* In Disney's Animal Kingdom® Theme Park (for the safety of the wildlife), balloons, straws and drink lids are not permitted.
I realize it says "not limited to" but if they were restricting food, it would definately say food.