I've looked around wordpress and from what i can see it looks like that searching by a specific category only is a function already inside the core of wordpress. By adding &cat=5 (5 category ID to search) on the search string in the url it should output posts only that matches that category ID.
This does not work for me on my 3.0+ site, plugins like search-by-category etc does not do anything.
What is wrong? Was this function exlucded in WP 3.0 or something?
I've tested with a fresh install and adding $cat=2 (example) renders the search result faulty (says nothing was found.)