Twitter is apparently finding that Rails doesn't do massive scaling well, 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 brings most of the great stuff about Rails while letting you get to the bare-metal if/when you need to.