[eluser]Jamie Rumbelow[/eluser]
Well, actually, if you look at it, CodeIgniter and Rails share a lot of similarities, and are both as heavy, in terms of workload for the server (Infact, Rails is heavier) and Twitter manages to run on rails with over 3 Million server requests an hour, or something like that.
No, CodeIgniter is not BUILT to handle extremely high-traffic sites, but that is not reason why it cant, with a little bit of optimisation.