There are some armies in 6th edition 40k that won't ally "Not Before The Apocalypse". Most notably, tyranids cannot ally with any other force in 40k. There are also other combinations (daemons and grey knights -- what? no heretic inquisitors?), but tyranids seem to have been singled out as the only army without any ally options. In some regards that isn't a surprise.
BUT.... if we're playing an apocalypse game, I'm allowed tyranid allies, right?!