Gruber's take. He caveats aplenty that he doesn't know any inside dirt but I think his speculation is of value:

The Mac is, and will remain, Apple's answer to what you use to do everything.

The Tablet, I say, is going to be Apple's new answer to what you use for personal portable general computing.

Put another way, let's say instead of a MacBook and an iPhone, you've got an iMac and an iPhone, but you also want a portable secondary computer. Today, that portable from Apple (portable as opposed to the iPhone's mobile) is a MacBook. With The Tablet, you'll have the option of a device that will more closely resemble the iPhone than the iMac in terms of concept and the degree of technical abstraction.

I think John's right on here. At my house I have a MacBook and in my pocket I have an iPhone. Hanging out watching TV, looking up some actor on IMdB, I'll use the phone instead of hauling over the computer.