While tracking down a presentation that Friends for Sale did, I ran across this excellent interview they did about scaling their backend at highscalability.com. It's fairly technical, but worth the read for anyone planning to handle 600,000 users a day (as I'm sure you all are).
Sample question from the interview:
What is your in/out bandwidth usage?
We used around 3 terabytes of bandwidth last month. This month should
be at least 5TB or so. This number is just for a few icons and
Yep, I just added 600,000 users = ~3 terabytes of bandwidth to my assumptions list in my budget projection, too. :)