Here is a good reading on comparison of MID (Mobile Internet Device) , netbook, and smartbook computers.
http://www.eetimes.com/news/semi/showArticle.jhtml?articleID=220100976&pgno=1
But Burchers has a clear vision for smartbooks' future: He sees momentum building around 7-inch tablets. Screen size is important, he said, because "more than 30 minutes of Internet surfing on a small screen like the iPhone will get taxing to consumers."Burchers expects smartbooks to offer many multimedia features, because "two-thirds of Internet usage by today's consumers is all about entertainment, or extracurricular activities, such as YouTube and Facebook."
Connectivity is critical for smartbooks, but constant 3G connectivity, which involves a monthly fee by carriers, may not be necessary. Eighty percent of users surveyed are happy with Wi-Fi only, Burchers said. Forward Concepts' Strauss estimated that 3G cellular capability could boost the hardware by $100.
According to Forward Concepts, of the 165 million notebooks, netbooks and smartbooks/MIDs expected to ship in 2010, 79.4 percent will be notebooks, 17 percent will be netbooks and 3.6 percent will be smartbooks or MIDs.
I also think that the screen size should not be too small. In order to surf website you need at least 800 x 600 resolution. And able to read the texts without your eyes getting tired. Maybe folderable screen would help.