From Armchair Generalist comes this useful statistical analysis of red-shirt survival rates. Of particular interest is the better odds faced by extras when there's an attractive (i.e. has pulse, somewhere) female in the vicinity. I am not sure if the odds improve as the show proceeds, although as noted below a fair number of walk-on casualties take place early on.
This also reminded me of Sigourney Weaver's great line (among many) in Galaxy Quest: "Let's get out of here before one of those things kills Guy!"
Laura and I saw the opening of an episode the other weekend in which not one but THREE redshirts were overcome by mist in the opening scene; two died immediately and the third died aboard the Enterprise. No girls on that planet for sure, even though the name of the episode is "Obsession." Lieutenant Klein to the fragrance deck, on the double!