WordPress MU

I'm trying to understand plugin section on MU.

I noticed something, even if in the blogs the plugin is deactivated, when I activate it on main blog, it works on all other blogs instead of only in the main blog.

I read the faq and stil don't understand the use of plugins on MU.

How do I install a plugin and activate it on a single blog only?

And in my installation, instead of create a new blog for the main site, I used the first blog "/", "wp_1", instead. Is that wrong?

I'm not sure what your last question means. But, activating a plugin on a single blog should work. You're not using the "activate sitewide" link, are you? Because that will do what you're describing - activate it for all blogs.

No, i was activating for single use and it works like a sitewide. The activation is made on the main blog, the first blog that you have before installation.

It shouldn't do that. What plugin is this?

I made my own, very simple. Lets see the code. Should I forgot to insert something in the code?

function me_get_category_template($template) {
	global $wp_query;
	$post_obj = @$wp_query->get_queried_object();

	if (is_single()) {
		$file = 'single';
		$categories = get_the_category($post_obj->ID);
	} else if (is_category()) {
		$file = 'category';
		$categories[0] = &get_category($post_obj->cat_ID);
	}

	$categories = me_get_category_parents_hierarchical($categories[0]->cat_ID);

	foreach((array)$categories as $category) {
		$template = "/$file-{$category->slug}.php";
		if (file_exists(TEMPLATEPATH.$template)) {
			include(TEMPLATEPATH.$template);
			exit;
		}
	}

}

function me_get_category_parents_hierarchical( $id) {
	$chain = array();
	$parent = &get_category($id);
	$chain[] = $parent;
	if($parent->parent) {
		$chain = array_merge($chain, me_get_category_parents_hierarchical($parent->parent));
	}
	return $chain;
}

add_filter('template_redirect', 'me_get_category_template');

and where did you put this "plugin". Code put into mu-plugins will automatically be applied across all blogs

I know and I put it inside /plugins, like the others.
I made a test, I just need to activate it on any of the blogs, just one, to let it work in all of them.

Here's what I did. I don't have any of the templates that you'd need to actually see what this plugin is doing. So, I added a dump of the $wp_filter['template_redirect'] global variable on the dashboard (via this code:)

add_action( 'activity_box_end', 'cets_dashboard_message', 1);

function cets_dashboard_message() {
global $wp_filter;
var_dump($wp_filter['template_redirect']);
}

Then, I put your code in a file with the appropriate plugin header information and activated it on the main blog. I correctly see these 2 variables:
["me_get_category_template"]=> array(2) { ["function"]=> string(24) "me_get_category_template" ["accepted_args"]=> int(1)

I went to the dashboard of a subblog, and the plugin-defined template_redirect variables are not in the template_redirect structure. Seems to work exactly as it should.

Are you using plugin commander or any of the other plugin manager things and somehow have it set to auto activate?