Show Posts that are in multiple categories

I am looking for a plugin or hack to do the following: given 2 or more categories, show all posts that appear in all of these categories.

This would preferably need to be dynamic (ie: I could go to example.com/categories/cat1+cat2/ and it would show me all posts that are in both cat1 and cat2). If it would not be dynamic, the second best way that I could think would be for me to define a new category in the admin, and assign the combination of other categories that define what displays for the new category (ie: I would define a new ComboCategory called Cat1Cat2, and set it to be all things that are in both cat1 and cat2).

I have searched the various plugin and support databases and have so far come up empty handed (there are ways to show posts from a list of categories ORed together, I haven’t found a way to easily AND them together).

Any thoughts?