Steve has made no such thing clear. Based on everything I've heard him say, I believe he imagines a world where the iPad becomes far more prevalent than PCs. Within the home, I think this makes some sense. The iPad is a great device for the sort of activities people typically use a PC for at home — casual web browsing, social interaction, and certainly games. It's portable, compact, affordable, and most of all fun. So where previously a family might have owned four PCs, that family might, in the future, own three iPads and one PC. Steve described this world by comparing PCs to trucks and tablets to cars — there are a lot more cars on the road, but trucks still have their place.
So, 'Post PC' is a term given to an era where touchscreen devices like the iPad start to dominate. It doesn't imply the death of the PC.
Does Steve really believe in such a world? Probably. But in a way he's also using it (as a marketing term) to try and create that future sooner rather than later — because Apple has the early lead when it comes to tablets, and it's a lead they dearly want to cement as quickly as possible. If they can get people to believe the iPad is the future, then some of those people might just hold off on buying that next PC in favour of an iPad, and I think Steve's confident enough that they'll be sold by the experience of owning one.
I predict (and you can quote me in ten years) that the Mac still has a long and healthy life ahead of it. Just don't expect Apple to pour the same amount of advertising into it as it does for iOS devices.
