I'm not in agreement with Denny on the starter gravity. In talking with Chris White (White Labs) many years ago, he said they culture their yeast using a much lower gravity wort. He indicated about 1.020, but I see that in his new yeast book, he mentions 1.030 as a prefered starter gravity. The issue of the Crabtree Effect is more pronounced when you increase the wort gravity, which diverts some yeast metabolic effort into alcohol production instead of yeast mass growth.
AFAIK all the yeast suppliers culture at <1°P (1.004) to avoid the Crabtree Effect entirely. The only reason that isn't a viable approach (pardon the pun) for most brewers is that you need to be able to add precise quantities of wort, and remove precise quantities of alcohol, continually through the entire culturing phase.
Martin's comment about keeping the flask headspace purged with air makes a lot of sense to me. I would think that even with foil or foam plug you would otherwise tend to have a nice blanket of CO2 on top of the liquid.
I did a sort of one-off experiment to see if that was the case, and it seems that you do get more growth with foil (or foam, presumably) than with an airlock. So I believe that diffusion is able to carry at least some O2 into the headspace to replace CO2.Aeration and Yeast Starters