http://en.wikipedia.org/wiki/Const
http://www.c2.com/cgi/wiki?ValueObjectsShouldBeImmutable
http://www.markhneedham.com/blog/2009/09/16/coding-watch-out-for-mutable-code/
Discuss issue of mutability from another aspect - http://www.artima.com/articles/hickey_on_time.html