People usually refer to Thomas Jefferson as the last Renaissance Man. It's been a long time since any one person could have a breadth and depth understanding of the world; one of those hard-to-verify factoids is that a person living today has been exposed to as much "information" (however you choose to define it) in a day as a person living in the 1800s saw in a lifetime.

So there will never be a reasonable case where a randomly-selected person will be literate about any given scientific subject. With any luck they'll have a basis of knowledge but it's a moving target; I'm not ancient but I'll bet you learned there were six kingdoms of in biology while I learned 5. I learned 9 planets, my kid will learn 8. The age of the universe has shifted by 50% since I first learned it (if I'm not mistaken - yet another example where "literacy" moves) and when I was a kid, Weekly Reader told me that I'd die of acid rain and no ozone layer. Nowadays it's global warming. I still remember when GRID became AIDS and it'd kill you really dead but Magic Johnson has been living with it for what? 20 years?

The important thing, from my point of view, isn't "literacy" per se but sophistication. We need people to be able to look at a news report and parse out that it's a sensationalist headline generated by one anecdotal study conducted by a grad student at an obscure university. We need people to be able to evaluate that one side of the debate is anchored by the World Health Organization and the other by Jenny McCarthy... and everything that means. We need everyone to grasp that just because there are two opinions does not mean they have equal weight.

It's not how much science you know it's how much science you can evaluate.