This may be a rumor but Techcrunch is reporting that Twitter is abandoning Ruby on Rails due to scalability issues. I have read a few articles a while back about people having problems with Ruby on Rails. That is why I was hesitant to use Ruby on Rails on some of my web applications (at work). I was hesitant so therefore never got to learn it either :(.
Personally, I think the best bet for Twitter would be PHP. I would trust my life with PHP especially with scalability issues. Techcrunch also mentioned Java, but I doubt Java would work for them. Remember Friendster a few years back? It had a .jsp extension in the URL and it was almost unusable. It was dead slow.
The developers of Twitter can write a clone within a day and no one would notice it. They should use the Zend Framework Anyways, PHP is FTW!