Obviously, Apple is not going to release just another tablet computer. I'm with John Gruber. It's got to be a game changer, and I think this could be the device to rule the digital home. People will also be use it to watch videos on trains and planes, read ebooks, and field workers might use it for data entry, but I think digital home might be its true calling.
It is going to build on innovations that they have been implementing elsewhere. As a starting point, we can assume that it will probably look more like an iPhone interface than the standard OSX interface. Multi-touch is a given, as is the accelerometer thing. Cloud computing will be an integral component. Third party apps will be heavily encouraged by Apple.
I could see this docking on your fridge door (and of course, being removable to carry around the house), to replace your paper calendar, post it notes, etc. It would do all the core stuff you would expect from a digital hub that is linked to some distant hard drive (in the home office, or in the cloud) -- calendar, view photos, control iTunes which outputs to Airport-expressed speakers, control AppleTV, look up recipes, check some fact on wikipedia, look up phone numbers, ...
Now the question is what else is it going to do?
I think third party apps will be encouraged to do things like control your thermostat and lights. People will use the device input their calorie counts or workout schedules or household budgets. Of course, apps will be developed for all sort of cool stuff around the house that we haven't even thought of yet. Maybe a built in camera will read bar codes and help manage groceries. Again, Apple will set the developer community loose to figure out how we can get value out of this thing.
"Where is my iPhone" will be a way to locate family members. You could see that being displayed on the fridge panel.
iChat could be integrated; maybe coinciding with this feature coming onto iPhones.
It might have 3G phone capabilities -- serving as a "base station" for your iPhone. We'll finally be able to get rid of our home phone, and people won't have to worry about being totally cut off if they loose their cell.
That's my 2 cents. The digital home is badly in need of a serious makeover -- and who else is going to crack this one open?