Updated Answer for Node.js ~>0.8
Node is able to concatenate buffers on its own now.
var newBuffer = Buffer.concat([buffer1, buffer2]);
Old Answer for Node.js ~0.6
I use a module to add a .concat function, among others:
https://github.com/coolaj86/node-bufferjs
I know it isn’t a “pure” solution, but it works very well for my purposes.