They have a close to 50:50 read/write ratio of their 100+ million page views a month.
If you were starting from scratch, would you consider going with Rails or would you choose something like C#/Java because you know scaling Rails to this level requires allot of tweaks of the framework?
800 requests per second is pretty high, not sure how many ec2 instances that would take with Rails.
Comments on your approach?
I realize you could probably serve 50% of your requests using varnish or memcache, so maybe it is very doable and you could benefit from the faster time to market and new features with Rails.
Note: The assumption is you will get 100 million page views, pretend this is for a re-write of an existing application for a company like Yahoo (please don't say worry about scaling later!) i.e. traffic/scale issues are real.