The only reason I would do web-dev in Scala wold be LiftWeb aka speedy comet-style apps, that is it's strongest point, otherwise I'd choose rails/django/whatever.

As each language seems to have a Sinatra clone, Scala's is Scalatra - for when you want to do something easy and elegantly-ish or something small as in "do this logic in a elegant fashion but don't hurt my brain". If you're stuck in Scala and LiftWeb is too much, choose the Scalatra's easy route.