Brilliant news for the entire node ecosystem! Having two competing implementations would not be ideal (even if they both support NPM). I hope under this new structure, the development will work at the speed of io.js. Well done to everyone involved!
They weren't really different implementations anywa - io.js was based on Node and most of the work they had done (as far as I know) was the move to the newer V8 engine version. So they were the same thing, just partially incompatible!
So it's really a great thing that they're merging, because it means that many more people can start actually using the new stuff!