I believe Twitter is suffering from its all too much praised "fail fast and cheap" approach.
The application itself is actually not that complex (check the Slashdot threads on those issues). By applying a generative process (RoR and MySQL) the application was shifting too much of the load balancing and scalability issues away from the database to a higher, persistence level. This is simply engineering architecture malpractice at its best. I wish people would shift away from blaming RoR and actually identify the underlying problem.
I have seen similar things happen over and over again: the misperception that the LAMP stack is a sustainable platform and ready for abnormal growth rates is proven wrong by Facebook as well.
I simply believe that this is an architectural problem that roots in an excitement for a new technology. Don't get me wrong: it's great they tried, but at some point you should better stick with best practices or it will become very costly.
It would be sad to see a great idea like Twitter die because some people fell for some nice marketing speeches.
Here's my advice: get an architect who knows best practices inside out and implement the new system design instead of just fixing the symptoms.