There is an apparent trend towards small laptops. I think there is a number of reasons for this trend.
OLPC project showed that it is possible to make a really small, cheap and robust computer that is usable.
There make less sense to increase performance (it is good enough) in computers and therefore price is becoming an important factor in competition.
People want to be mobile.
Internet are becoming more important for communication and amusement.
Price is very attractive. These computers sell for around 500 USD while alternative (MacbookAir, Lenovo X61) full configured laptops cost around 2500 USD.
Requirements
My main requirements for a mini laptop is;
Small and light
Reasonably usable keyboard
Robust to handle mobile use. Preferably SSD hard disc.
Screen size of at least 1024 pixels
Light i.e. around 1 kg
Good support for different OS alternatives. For me in particular OpenBSD.
Alternatives
We are at the beginning of this trend but there are some alternatives to choose from. Available now or in a short time frame.