Filter posts under multiple categories?

How do I filter the posts by multiple categories? For example, if some of my posts are categorized under just apples and some as just oranges and some both, how would I filter the results to show only posts marked under both apples and oranges?

In other words, I don’t want the results to list all posts under apples and all posts under oranges. I only want to show posts listed under both apples and oranges. Is there a way to do this?

WordPress does this automatically if you do

http://yoursite.com/?cat=1+2 (where 1 and 2 is category ids)

OR

http://yoursite.com/?category_name=apples+oranges (where apples and oranges is the slugs)

OR something like

http://yoursite.com/category/apples+oranges