See http://arevico.com/change-the-template-for-all-posts-in-a-specific-category/ on how to use this gist.

wordpress-in-cat-template.php

<?php
/**
 * This code is intended for both theme developers and webmasters. It allows you to specify a page template
 * for any post belonging to a specific category.
 *
 * The script looks for a file called in-category-{$slug} and if it exists and uses that file as a template. Replace {$slug] with
 * the friendly name of the category (slug)
 *
 * @author H.F. Kluitenberg < http://arevico.com/ >
 * @usage See http://arevico.com/change-the-template-for-all-posts-in-a-specific-category/
 */

$inCat = new InCat();

class InCat
{

	/**
	 * Check if the current post belongs to a specific category if a template has been found return it.
	 *
	 * @param string $template The full location to the template
	 */
	private function inCategoryTemplate( $template ){
		global $wp_query;
		$availableTemplates  = wp_get_theme()->get_page_templates();

		if (!$wp_query->is_singular)
			return $template;

		$catNames 		= array();
		$cats 			= get_the_category();

		if (empty($cats))
			return $template;

		foreach( $cats as $cat)
			$catNames[] = preg_quote($cat->slug);


		foreach ($availableTemplates as $path => $name )
			if (preg_match('/in\-category\-()' . implode('|', $catNames) . '/', $path))
				return get_template_directory(). '/' . $path;

		return $template;
	}
	
	/**
	 * Constructor. Adds all required hooks and filters
	 */
	function __construct()
	{
		add_filter( 'template_include'	, array($this, 'inCategoryTemplate'), 99 );
	}
}