I think the only place in Disney that truly has a dress code is Victoria & Albert's. Yes, the others have a dress code listed, but as you've probably seen the only things I don't think are allowed are cut-offs and torn clothing, swim suits, and tank tops. I think even if you did just have flip flops, you'd be fine. Most of the places seem to allow for the fact that a lot of people will be coming to the restaurants from the parks. I also have this excerpt from the Disney Food Blog:
The official dress code for these [Signature] restaurants states:
Men: Khakis, slacks, jeans, dress shorts, collared shirts. Sport coats are optional.
Ladies: Capris, skirts, dresses, jeans, dress shorts.
Not permitted in dining room: Tank tops, swimwear, hats for gentleman, cut offs, or torn clothing. While T-shirts are now allowed, the policy remains that T-shirts with offensive language or graphics are not acceptable.
In practice, this means that guests are dressed just one step up from what they would wear to the parks. Although some guests choose to wear business casual or cocktail attire to signature restaurants, you will often see at these venues wearing khaki shorts, a t-shirt and sneakers. Men should avoid tank-style or muscle shirts. Women may wear sleeveless tops or sundresses as long as they not overly revealing. There are no prescribed footwear requirements, other than that you must be wearing shoes. Specifics of the dress code are not strongly enforced. As long as you are neat and presentable, you will be seated.
The signature dining restaurants located within the theme parks (Hollywood Brown Derby, Bistro de Paris) have no specific dress code other than standard park touring attire.
