As an aside, one of the really interesting ideas that's bubbling up from Rust is that it's not mutable state that's a problem, it's shared mutable state, and that you can make it safe by removing the sharing, rather than removing the mutability:

http://smallcultfollowing.com/babysteps/blog/2014/05/13/focu...

I have no idea if this is true, but it's going to be really interesting to see that idea tried.