We all knew it was coming (unless you have been living under a rock that is), but this seems to be the first piece of concrete evidence indicating it as reality. Allies are here.
Now to be fair, WHFB 8th edition has always had the allowance to field allied contingents, however they have always been regarded as one of those 'back of the book' options that nobody chose to play. But with the advent of battlescrolls, itself a large shift in the structure of WHFB, it is spelt out.
Could this be the end of the percentage based army selection? Will 9th usher in a new area structured closer to 40k. It is not hard to jump to conclusions based on the above limited information, but indeed Battlescrolls and battlesheets draw a lot of similarities to primary and secondary/allied detachments. Specific battlescrolls easily representing the equivalent of a special formation in 40k.
At first glance, I can see a lot of advantages to this format, indeed putting further restrictions on overpowered units and even balancing them out by the requirement to field certain inefficient choices to take a formation. but then again, no doubt GW would throw in the 'unbound' battlescroll, or take whatever you like, ok I guess for friendly games (bring all your models!).
Only Tzeentch knows, the next 6 months will be a rough ride for some...