Speaking with the Fast Company, Valve's marketing gaffer Doug Lombardi has confirmed that more content is on the way, possibly as soon as this summer. "We will be announcing some DLC for it, so folks can keep a look out for more content coming this summer." Hurrah.
If the summer still feels like a long way away, Lombardi has confirmed that Valve will be releasing details about the content "in a week or so". A fresh batch of co-operative puzzles would suffice, Valve. Another ten hour single-player campaign wouldn't hurt either, would it? That's not expecting too much, we don't think.