It was released for Xbox on September 4th and allows you to build your own house from scratch (in Falkreath, Hjaalmarch or the Pale) and adopt children.
Here's the trailer: http://www.youtube.com/watch?v=TvhQZoyZn1g
If you're on the PC, here's a fan-made mod to play with while you're waiting (though it doesn't let you adopt kids): http://skyrim.nexusmods.com/mods/18480
No announcement has yet been made (AFAIK) about release dates for other platforms, though I imagine it will be out quite soon.
How strange indeed. Some places say that it only adds three plots of land which you can build on, which wouldn't interest me. Then somebody was talking about adding a new wing to Breezehome, which got me all interested. The pinned faq post makes no mention of being able to expand your existing homes, so here I am, doing "quick research on the net and this forum".
You cannot expand existing homes, but you can add a child's bedroom to them. That way, if you want kids but like Honeyside, you can raise your family there.