To handle the peak rates of Tweeting, 5000 per minute during the Obama Inauguration [1], Robey Pointer at Twitter has moved their main message queue from Ruby to Scala for improved performance. He explains the concepts behind the Twitter Kestrel project [2] on his live journal [3] and the more concise 1500 lines of Scala code can be seen as open source project here [4].
Links:
[1] http://blog.metaroll.com/2009/01/22/almost-5000-tweets-per-minute-during-obamas-inauguration/
[2] http://blog.twitter.com/2009/01/building-on-open-source.html
[3] http://robey.livejournal.com/53832.html
[4] http://github.com/robey/kestrel/tree/master