Forbes has an interesting article about how Asus developed the Eee PC. Asus' CEO Jonney Shih was looking for a breakthrough product about a year ago to give Asus another boost.
Apparently the software development took the longest time. Getting the Linux based User Interface right was important and it was the reason for the delays.
Microsoft got their foot in the door at the last minute, so now there will be Asus Eee PCs that you can have with Microsoft Windows for an ultra-low $40 license fee.