I know its been a while since this topic had a post but thought I should raise this....
There is fluff in the book about the leader of the BT being captured by the Storm Lord and then having his hand cut off by the Storm Lord instead of being killed.
Also the reason for the BA teaming up with the crons is a little more clear now.... The Necrons were led by the Last of the Silent Kings, who was returning from self imposed exile to raise his forces against the Tyranids. The reason he wanted to destroy the Tyranids is that if they consume all life in the universe then what bodies would the Necrontyr use to be reborn in flesh!!
The new issue of WD has a whole campaign relating to the war between the BT and the Newcrons. Could be interesting!