I decided to switch to WordPress so I could have categories in my blog and I could get RSS feeds on only one of them to be imported in different planets… But I had posts in both French and English in my Gentoo category. So I check if I could say “only take posts from category english and gentoo”… But if I select two categories, I get a union, not an intersection. So I wrote this little plugin. To have an intersection, you can do ?cat=x,y&intersect=1 … Example http://www.tester.ca/?cat=2,10&intersect=1 for my English & Gentoo posts. Also, this only works with category numbers.
File is here, dont forget to rename it .php to remove the .txt extension. Put in in wp-content/plugins/ and enable it..
Update: I just upgraded to 2.5 and I think it works as is. Please report any problems