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.
