“Twitter is apparently finding that Rails doesn’t do massive scaling well”:http://www.radicalbehavior.com/5-question-interview-with-twitter-developer-alex-payne/, at least not the way that all the books will tell you to write stuff for it.
That doesn’t really surprise me. Making applications that scale well is _hard_. I’ve done it twice (though only one of those is a web app), and in both instances, what I found was a need to be able to muck around with the lowest-level code to be able to create app-specific speedups–whether that was writing my own hand-tuned demented-but-fast SQL or being able to back stuff up against memcache that most people wouldn’t think to put in there, like mutexes (and yes, I know it’s not a reliable storage medium, but given the rate at which it fails, we were willing to face potential issues).
And, honestly, I think “Catalyst”:http://catalyst.perl.org/ brings most of the great stuff about Rails while letting you get to the bare-metal if/when you need to.