Just read this interesting post on Raspberry Pi's site with some great answers from Pete Lomas:
A couple of weeks ago, I had dinner with Pete Lomas, the Raspberry Pi Foundation trustee and all round good bloke who did the hardware design for the Raspberry Pi.
The reasons for several design decisions are explained including ADC which had been on my mind. I really like Pete's reply to "How to be so awesome?":
"keep focused and you’ll get there…eventually. That’s what I kept telling myself trying to route out the BGA…