with my 7100i I have them categorized. I put the restaurant name in the first name field. Then I put "restaurant" into the company field. when I type restaurant, everything having restaurant comes up. I dont know if this works with other models, but try it.
Create a category for resturant snd assign it to each restaurant entry in your contact list. Then to see only restaurants you can filter on that category from the address book menu.
You can even create categories for different types of restaurants.