The all-new Ford big block has been under development for several years now. At one point it was canceled, then later restarted. Earlier this summer, all work to put versions of it in the Mustang and F-150 was permanently canceled, with only use in the Super Duty a possibility going forward.
So it's dead for Mustangs - leaving Ford out of the "big block" world. Instead, Ford will use a variant of the current 4.6 DOHC engine in 5 liter DOHC form in the re-skinned 2010 Mustang (or a year later, depending on corporate finances) and a version of the twin-turbo V-6 "EcoBoost" 3.5 liter DOHC V-6 (in 2010 or later, again depending).
The only picture ever published with permission of the dead big-block was in a Roush drag Mustang, in iron-block SOHC cam form:

However, some other pictures of this same iron-block SOHC engine made it out the doors:

Now that the engine is dead, will there a plan put in place to offer it for off-road Mustangs via the aftermarket? Speculation at this point, and given it's weight and lack of factory support not particularly desirable. We think this is the end of this engine for the Mustang crowd...