A while ago Alex McGuire gave us an insight into Scala at EDFT, now Lee Momtahan goes public talking at the Commercial Users of Functional Programming meeting, CUFP 2009 in Edinburgh , about their experiences at Électricité de France Trading  (EDFT). He is a member of a group developing applications for energy derivatives trading and pricing. He explains the benefits EDFT has gained by replacing a significant part of their 300,000 lines of Java code with Scala, DSLs and their impressions of Scala. You can find a summary of his talk here , his slides here  and a video of the actual presentation here .
He summarizes by saying "We're using Scala for real work in a business critical situation and there is a lot of money at stake if we get it wrong. There are risks, but we have had no problems [with Scala]. For me a developer working at the coal face, the productivity improvements are massive."