"Both the A57 and A53 employ the 64-bit extensions to the ARMv7 architecture that were announced when the 64-bit ARMv8 architecture was unveiled last October at that year's ARM TechCon."
ARMv8 is not an extension to ARMv7 by any means. It is in fact an entirely different instruction set architecture (ISA) that is not at all compatible with the previous generation. This is very different from, say, x86_64 or PPC64 that are extensions to their predecessor ISAs.
"The A57 is essentially a Cortex-A15 with the addition of the ability to handle 64-bit processing,"
Yes, except not really. Some parts are similar, but looking at the new ISA, ARMv8 is actually quite a bit more RISC-y than ARMv7-A, doesn't use predicated instructions, omits complicated multiple load/stores, and is in general better designed for parallelization and reordering by a modern CPU. Its pipeline would be quite a bit different than an A15 with its relatively complex ARM and variable length Thumb instructions. It's far more likely that the A57 is a 64-bit processor with backwards support for V7-A.
"Expect A7/A15 mashups using big.LITTLE to appear in products next year, they say."
It's nice to see that hardware designers still don't understand how software actually works. While there may in fact be chips that sport fully coherent A15+A7 complexes, the number of products that actually use them as ARM imagines will be countable on zero hands. Ask Nvidia about the Tegra 3 and its companion core for details on why this doesn't work in the real world.