As I said in the article, it's event based which means it can handle massive concurrency and serve pages that don't necessarily reply straight away - perfect for implementing things like comet. It gives you similar capabilities to Ruby's EventMachine. I don't think PHP has an equivalent at the moment.
To me, the important part of node.js is that it's using the V8 (http://code.google.com/p/v8/) JavaScript engine which Google has a strong, vested interest in being the fastest dynamic platform around and a lot of really smart programmers to make that happen.
Otherwise, it's a matter of preference. What's the main advantage of PHP/Ruby over Perl?