It's not. According to Rowling (and the publishers) it'll be out the summer of 2005 at the earliest, most probably the summer of 2006.
When Order of the Phoenix was to be released in mid-June, they had announced it in January. We don't even know the
name of the next book.
I think, if I'm right, then some major reorganizing of the school would have to be done. But I think that desperate times call for desperate measures, and we're in desperate times at Hogwarts.
So, yes, I think adding a bed to the sixth year dormitory would be done (incidentally, I think the setting up of the dormitories is magical anyway, because I don't think there's always 5 boys in all the Gryffindor boys classes)
You raise a good point about Dumbledore wanting Malfoy having intimate access to Harry, but since Lucius is in Azkaban, and since there would only be one of Malfoy and LOTS of good kids in Gryffindor, they're more of a danger to him, not the other way around. I think Dumbledore would also warn Harry, Ron, Hermione, probably Neville and Ginny on the side to keep an eye on Malfoy, but don't let him hear or see too much.
I am not saying that it won't happen but I just don't see how (how did Hermione put it in the trailer?)
a foul loathsome evil little cockroach should get the privilege of going to Gryffindor.
First, he would see it as some kind of punishment, since he loves being in Slytherin. Second, you think it's better that the foul loathsome evil little cockroach stick around with his buddies and cause a real problem in Hogwarts? Have you ever tried killing a drawerful of cockroaches, instead of just one? It's a LOT easier to contain the one.
If it were to happen the way you say then Snape would be in a bit of an awkward situation as well. Will he continue to take points from Gryffindor?
That depends on if the entire house of Slytherin is disbanded, or if it's just the little inner circle. If it's just the inner circle, then Snape would continue being the head of house, taking house points away, etc. If the entire house is split up among the other three houses, then there wouldn't be a need for a head of Slytherin house, so, unfortunately, Snape would lose a part of his responsibilities at Hogwarts.