WordPress
in_categoryはカテゴリIDのみでループの内側でなければならないという記述がよくあります。
しかし、WordPress2.7以上ではスラッグが使え、ループの外側でも使えるようになっています。
Function Reference/in category
>Notes
Since Version 2.5, you can specify categories by name.
Since Version 2.7, you can specify categories by slug.
Since Version 2.7, you can check against several categories.
Since Version 2.7, you can use this function outside the WordPress Loop (during a single post query, presumably)
Since Version 2.7, you can specify any post to test (not just the current one).