I've always wanted to own my notebook company so that I can make notebooks the way I think they should be made. I really like the new Ultrabook category and feel that most of the companies should start putting more focus on those type of notebooks than their mid-range lines.
If I were to define categories for where I would concentrate on mobile computing (other than phones), I would do these 3:
1. Tablets: Less than $500
2. Ultraportables (Ultrabooks): Less than $500-$1000
3. Power Notebooks: $500 and over
There is a case for a 4th category, Budget Notebooks, but I don't think the profit margins would be high enough to support it (ie... let someone else have that headache).
Number 3 is actually a wider category when it comes to form factors, size, weight and price but the idea is they are the multi-media, gaming and high-end graphics machines.
You will notice the lack of netbook. That should be serviced by either a tablet or an ultrabook. If you're using a netbook for consumption, a tablet will do... if you are using it for creation, that's where an ultrabook will do more than what a netbook could. And you really could do 11" ultrabooks and 13"/14" ultrabooks. In fact, I would consider adding a 15" ultrabook sub-category to be able to give Full HD screens to the ultra thin and light crowd.
Coincidentally, one prominent and profitable maker is set up like this... Pad, Air and Pro. Choice is great, but there is something to be said about a streamlined run of notebooks that fills each users need as much as possible.
If I had to sub class each category by screen sizes, it would probably be something like this:
1. Tablets: 7"/8" and 10"/11".
2. Ultraportable: 11", 13"/14" and 15"/16"
3. Power: 15"/16" and 17"
Seems like that could fit the majority of mobile users out there.