First, they have a functional neuro-electronic interface able to provide coherent signals. That alone, unless I've missed something, is a major breakthrough.
Second, they've been able to get individual neurons to self-organize to the point where they can cause observable behavior and respond to external stimuli.
Third, they've not only shown that a small collection of neurons, on the order of 100,000, can learn, but they have been able to influence it's learning and behavior by chemically altering individual or small groups of neurons.
Just the research avenues this allows are significant. The computational possibilities are astounding. The medical/enhancement possibilities this opens up are, no pun intended, mind-boggling.
I have a healthy skepticism of some of the more out there claims of the singularity-ists, but this is not just a move in that direction, it is an olympic-sized leap.
UPDATE: A better article with more detail