Scala School!
Scala school was started as a series of lectures at Twitter to prepare experienced engineers to be productive Scala programmers. Being a relatively new language, but also one that draws on many familiar concepts, we found this an effective way of getting new engineers up to speed quickly. This is the written material that accompanied those lectures. We have found that these are useful in their own right.
The link address is: http://twitter.github.com/scala_school/