That's right, the sensor is for sale. We haven't made an announcement about it yet because we're still trying to polish the example programs we want to release with it. However, if you're feeling adventurous, you can purchase it! Xander has published a great example of how to make it a webserver and control it through the internet already.
The hardware is working great, and we have provided documentation from the manufacturer on how the wifi module itself works.
When we finish up some examples, we will do a grand release. Until then, it can be purchased here: http://store.dexterindustries.com/di-wifi-for-lego-mindstorms-nxt.html
And you can see and unpolished start of a user-manual here: http://dexterindustries.com/manual/wifi/