I thought I heard about there being a cork shortage as well, after googling around it seems that this may be an urban myth. Even Cork conservation sites say there is no cork shortage [1]. Looks like twist-offs/plastic corks etc. may just be around because they are cheaper than natural corks at current market prices. Would be interesting to see how the production and demand for plastic vs. natural corks swings with the price of petroleum or other chemicals necessary for plastics production.