Actually my experience (iirc) is that luxury resources takes one turn to get access to. I mean - you settle a city on top of a luxury, happiness goes down by 3, then only in the next turn it goes +5 (of course if it's a new luxury). On iron though it's instantaneous - you plop down a city and can upgrade warriors to swordmen in the same turn. Also, if you settle a city on a resource that you don't yet have the technology to access to (mining/calendar), then the resource becomes available immediately (=in the same turn) when you research that technology.