I'm impressed with Twitter and investigating to use Scala for a new large scale web project with Hibernate and Wicket. What do you think about Scala, and should I use it instead of Java?
EDIT: And, do you think Google's Noop, Fan or Scala can take the leadership from Java in the near future, which one have chance in your opinion?
Don't forget that you can use it alongside Java. i.e. you can make use of compiled .classes and .jars within a Scala-scripted environment.
So you may want to encode the lower-levels of your solution in Java and then wrap these in Scala to provide a more flexible and agile solution (in terms of rewriting business logic at a higher level).
Something to consider is who else is going to look after this and maintain it. For all of Scala's hype, relatively few people can code in it (as of Nov 2009), and you may have to hire people in the future to look after it. Looking for Scala people will mean you're looking in a much smaller pool of candidates (some may want to learn Scala on the job, but there's still a start-up cost)