We do have RSS feeds on Tags but you have to be a "recent" sort, instead of the default "relevancy" since in RSS it makes a bit more sense (at least to us) to have things based on their creation time so that if you monitor a tag search of "Pets" you'll always get new questions asked on it.

If you'll go to http://yedda.com/questions/tags/pets/ you'll see the tag search for pets.

If you'll go to http://feeds.yedda.com/questions/tags/pets/feed/?format=rss2 you'll get the feed for that same tag search (you could also simply change to recently asked sort and if you use FireFox/Internet Explorer/Opera that recognized feeds on a page you'll see the feed.

Regarding full text search feeds, we will look into that.