Morning callbacks typically include lunch, although it's not written in stone. They'll usually tell you beforehand, but not always. Just be prepared for it if it happens (although if you find out ahead of time and really don't want to do it, you can always opt out).
Some tips for the lunch:
Never order alcohol (a glass of wine at dinner is ok so long as the other attorneys are ordering)
Don't order appetizers or dessert unless the other attorneys do.
Never order the most expensive thing on the menu--order something in the mid-price range (again, follow the lead of the attorneys here)
Stay away from messy food--ordering spaghetti with meatballs, for example, will almost always result in you spilling sauce all over your suit (a form of Murphy's law)
Although the attorneys who take you to lunch (usually young associates) may encourage you to ask candid questions (i.e. about pay, etc.), don't do it. You can be slightly less formal with them, but stay on your guard. They will be filling out evaluations after the meal.