Rails, and the alternatives mentioned don't really make sense for a messaging application. You wouldn't write the back end for Gchat or AIM in Rails (or PHP, or Django), either. Rails would be fine as the web front end that talked to another messaging system. I've interviewed with the twitter guys and been to their talks and they definitely are NOT doing the right thing. However, "the right thing" is not easy and there are few people around and available who can do it.
edit: their best bet is to try to find someone who is burned out from working on wall street applications.
Twitter isn't a messaging application in the traditional sense (as in IM or phone networks) because it doesn't involve real-time synchronous communications. All communications in Twitter are asynchronous as in most database-backed webapps and I think it should be possible to scale Twitter with Rails assuming you know how to scale the database tier.
Having said that, it would still be fun to rewrite Twitter in Erlang... :)
Dang - I was going to say Erlang. This is an ideal application for Erlang... scalability and uptime. There's a reason it's heritage is in the teleco industry.
I went and had a look at the slides for their 'scaling twitter' presentation, and it looks like they had a look at Erlang and reacted with some silly "ooh, it looks ugly" sort of comment. Seems pretty silly to reject something out of hand that is very much along the lines of what you need.
edit: their best bet is to try to find someone who is burned out from working on wall street applications.