7th edition has some big changes including Tyranids now being able to ally with other forces. In fact the allying matrix has changed a lot and "come the apocalypse" on the chart, still allows you to ally, since this is.... well, the end of times with the apocalypse at hand.
Still consider these rumors as we have not seen an official release yet.
According to this, Come the Apocalypse allies will have some serious penalties.
via an anonymous source on Faeit 212
Here is a link to the leak
http://www.lounge.belloflostsouls.net/showthread.php?39686-Warhammer-40k-7th-ed/page54
Armies of the Imperium
Battle Brothers: Armies of the Imperium,
Allies of Convenience: Eldar
Desperate Allies Dark Eldar, Tau
Come the Apoc: Everything else
Chaos Daemons
Battle Bros: CSM, Chaos Daemons
Allies of Conv: None
Desperate Dark Eldar, orks,
Apoc: Everything Else
Chaos Space Marines
Battle Bros: Chaos Daemons, Chaos Space Marines
Allies of Conv Necrons, Orks
Desperate Dark Eldar, Tau
Apoc: Everything Else
Dark Eldar
Battle Bros: Eldar, Dark Eldar
Allies of Conv None
Desperate Orks, Tau, Armies of the Imperium, Chaos Daemons, CSM
Apoc: Everything Else
Eldar
Battle Bros: Dark eldar,
Allies of Conv: Armies of the Imperium, Tau
Desperate, Orks,
Apoc: Everything Else
Necrons
Battle Bros: Necrons
Allies of Conv: CSM, Tau
Desperate Orks,
Apoc: Everything Else
Orks
Battle Bros: Orks
Allies of Conv: CSM,
Desperate, Daemons, Dark Eldar, Eldar, Necrons, Tau
Apoc: Everything Else
Tau
Battle Bros: Tau
Allies of Conv: Eldar, Necrons
Desperate Armies of the Imperium, CSM, Dark Eldar, Orks
Apoc: Everything Else
Tyranids
Battle Bros: Tyranids
Allies of Conv: None
Desperate: None
Apoc: Everything Else
