Several months ago, it was somehow a little tricky to install Node.js and npm on Debian systems. Now it has become as easy as it could be with the help of APT pinning. (Be careful with APT pinning, since it may break your system.)
Now you can install the latest stable version (v0.6.x) of Node.js with
apt-get -t unstable install nodejs
and npm with
curl http://npmjs.org/install.sh | sh
And that’s it! By the way, it’s a little confusing, that the latest stable version of Node.js is in the unstable distribution of squeeze. But that’s the Debian policy.