Don't use JavaScript for numerical computation. Don't block your IO loops for CPU intensive stuff.
commentary:
I've never met a Node hacker who would ever do any of those things. This post should be called "Straw man Node.js n00bs jump the shark."
I've written web apps in a wide variety of dynamic languages, and you know what, I've never done anything CPU intensive in the request response loop, ever. This has nothing to do with Node, fibonacci in PHP is just as bad an idea (although it might take you a little longer to realize it, as you've probably got lots of PHP threads running behind your Apache).