One of the cool things about PS3 is that you can assign products to multiple categories. This way you can have complete control over what items the user sees when they click on the main category.
So you can assign all of the sub category items to the main category or just the ones that you would like the user to see when they click on the main category. You have complete control over this. To assign a product to more than one category you just need to hold the Ctrl key when clicking on multiple categories.