15 Inch != Ultra
Personally I would put the limit at 14" for anything to be called Ultra. 15 inch starts to become difficult to put into a certains bags/briefcases, which already have papers/notepads/books in them. 15" is a large piece of equipement, 14" just passes the mark.
Even if the thickness were reduced down to 1cm for a 17" its sheer area stops it from looking portable, 17" remains in the transportable range.
15" is the same it doesn't belong in the Ultra range, it will always belong in the portable range due to its surface area
[I wont bother moving into the Ultra/Ultra range or the Ultra Netbook as I am sure that you have already caught the drift ]