If you really want scalability take a look at Erlang though ErlyWeb (erlyweb.org/) is not that mature, Yaws (yaws.hyber.org) really is. Ruby and Ruby on Rails are really great for development i had never been that productive. Rails seems to scalo quite ok wenn adding more servers and rewriting som/lots of code. In an feature product I seed more safety and scalebelity so I will implpmpnt it in Erlang