Wired has a great article by Pete Lomas, a co-founder and trustee of the Raspberry Pi Foundation, where he describes the design process for the Raspberry Pi.
http://www.wired.com/opinion/2012/09/raspberry-pi-insider-exclusive-sellout-to-sell-out/?pid=45&viewall=true
http://www.wired.com/opinion/2012/09/raspberry-pi-insider-exclusive-sellout-to-sell-out/?pid=45&viewall=true