A sub-notebook (ultra low cost PC - ULCPC, or netbook) attempts to deliver the promise that laptops have mostly abandoned. This promise is a real portable computer that can be used on the road for extended periods and is small enough to not weigh you down carrying it.
The Sub-notebook and ultra-mobile's have the same claim to fame which is that contrary to a regular laptop they can actually be used on the road. For that they need to have a great battery performance and be light-weight. Usually this combination comes at a premium: rare are the ultra-mobiles that come cheaper than 1000 USD. The Sub-notebook differs primarily from the UMPC in that it does not run a tablet PC style OS and does not usually have a touch screen. A Sub-notebook differs from a Palmtop device in that it can run a bigger OS and can, in some cases, really replace the Desktop computer in the field by being able to run the same applications.
Recently a number of ultra low cost PC laptops have been announced that have potential as eBook reading devices. These new devices complement the already announced educational laptops for developing countries, the OLPC 2B1 and the Intel Classmate. The new devices are intended for bulk sale to governments but they can also be bought by the general public.
This new generation of devices promises to deliver these same features, at the cost of other features, and be much cheaper.
 Netbook Restrictions
Both Microsoft and Intel have limited some of the specifications on Netbooks that use their products. In particular Intel Atom 270 and Atom 280 processors both using Intel 945GSE graphics accelerator are available only on devices with a screen size no larger than 10.2", screen resolution no more that 1024x600, with memory no larger than 1 Gig and disk drives no larger than 160 GBytes. Of course, the user may be able to upgrade the units after purchase. These limitations are likely to change in the future. (Originally Windows XP was required but the advent of Window 8 this has changed.)
Many of these devices use the wide screen format adopted for TV (16x9 aspect ratio). Using this ratio an 8.9" diagonal screen would be about 4.4" high and 7.8" wide. A 10" to 10.1" screen was measured and has the dimensions of 8.7" x 5.1" which is just a tad higher than 16x9 would be. With a screen resolution of 1024 x 600 it would have a ppi of 117.