I won't go into too much detail here because ReadWriteWeb have done it better. Also of interest, Felix Geisendörfer (a former CakePHP contributor) also explains why he's more interested in Node.js now.
The most interesting point for me is this: JS is currently getting a lot of attention (read millions of dollars) from many of the biggest players in the industry: Google, Apple, Mozilla and Microsoft. That means it is being optimized and getting faster at a rate that other server side languages can only dream of.
I've heard (but not verified) that some bits of Node.js' C++ runtime have actually been re-written in JS because there was no performance difference!
Now I love JS as a language, it is getting seriously quick, and we are getting closer to having a stable and relatively mature stack to write applications with. It probably isn't going to power the next twitter but this is certainly worth looking into.