Not that this is a huge surprise, since we were already expecting next year's phones to arrive with 64-bit processors, but Samsung wants to make it clear, just in case, that their phones will have 64-bit chips next year, too.
I've already said why Apple managed to move to 64-bit so quickly – they took a shortcut by moving the "old" Swift processor to the ARMv8 architecture, so they didn't "waste" any time creating a whole new CPU design, too, which means Swift is not the first "native" chip for the 64-bit ARMv8 architecture. The Apple A8 chip will be.
Apple – especially Apple – won't gain much from it, either, considering they are usually at least a generation or two behind other manufacturers when it comes to using more RAM in their iOS devices. Their new iPhone will most likely use 2 GB of RAM (at most), if not even the same 1 GB as last year. So it will be a while until Apple starts to take full advantage of its own 64-bit chips.
On the Android side, things are bit different. We're already seeing companies adopt 3GB of RAM in their high-end devices, something Samsung is doing, too, and normally I'd expect 4GB to start being adopted late next year, just like we saw 3GB late this year. However, since Samsung seems to be so aggressive about announcing 64-bit, I'm now almost certain that the Galaxy S5 will come with 4GB of RAM.
It will be a little early for 4GB adoption, but it won't be that early. Samsung is already a leader in mobile RAM, and they certainly have that capability to use 4GB of LPDDR3 RAM in their next flagship,by next May. It will just cost them a little more to have it ready this early. The reason I think they will do it is that unlike Apple, they will want to say that their phones don't support 64-bit "for nothing", and the 4GB of RAM will actually require it.
Now, there's also the question of whether this will be Samsung's own CPU core, that has been rumored, or it will be another "Octa" chip with Cortex A57 and Cortex A53. If the rumor about Samsung making its own CPU core is true (not that unlikely anyway), then it's most likely going to be the one used, since the Cortex A50 series won't be ready until late next year. I'd also like Samsung to use its own chip instead, since that brings us more competition in the ARM space. Hopefully it will be made at 20nm, instead of 28nm. Samsung owns its own fab, so that's not unlikely either. It will be 2 years (a generation) since we saw 28nm, so 20nm is expected.
Another question will be whether Android itself will support 64-bit by then. If Android 4.4 KitKat comes at least with Linux kernel 3.8, then it should, unless Google specifically ignores that support in the kernel. However, I also think Google needs to make changes to the Dalvik VM, so we'll see how it goes soon enough, when Google unveils Android 4.4.