My idea for achieving optimal content crawlability and SEO optimized URLs is to use permalinks instead of ids and the default Rails routes. The permalinks can contain whatever you decide is optimal from a SEO point of view.

As an example, let's take a recipe site that has a recipe at http://xxx/recipes/asia/china/beijing-duck.html.

First let's configure the .html extension to be handled by the RecipesController:

map.connect 'recipes/*permalink.html', :controller => 'recipes', :action => 'show'

In the code we use the URI, which is the permalink of the recipe, to retrieve the recipe from the database:

class RecipesController
  def show
    @product = Recipe.find_by_permalink(request.path)
  end
end

To handle the categories and subcategories, we use the following route:

map.connect 'recipes/*permalink/', :controller => 'categories', :action => 'show'

And create the CategoriesController:

class CategoriesController
  def show
    @category = Category.find_by_permalink(request.path)
  end
end

Now what's left is for you to figure out how to generate the permalinks... I recommend having a look at permalink_fu.