Keep in mind that many PCs are already capable of running 8 threads simultaneously, since Intel has been selling quad-core CPU's with multi-threading for some time. From software's perspective, a machine that can support 8 threads looks just like it has 8 processors to work with (which is why if you open Task Manager on a 4-core machine with multi-threading, you'll see 8 logical cores). So there shouldn't be any issue with taking software that's optimized for 8 cores and porting it to existing PC's (as long as it supports 8 threads).
FYI, folks need to be careful about what AMD calls a core.
I haven't looked into the PS4 announcement, but AMD is currently selling what it calls an 8-core processor that actually consists of 4 cores, each of which have two separate execution engines. Their marketing is calling those execution engines separate cores, even though they are fed from a single instruction issue queue.
It's basically AMD's way of implementing multi-threading, in an attempt to counter Intel's multi-threading implementation (Hyperthreading), but they're doing it with a much higher hardware cost, albeit with a theoretical increased performance.
However, benchmarks have shown that there is very little performance benefit (if any) of an AMD 8-core processor when compared to a 4-core Intel processor that implements multi-threading (ie, each core can run 2 threads concurrently). This may be due to other micro-architectural differences, though; Intel's pace of new micro-architectural releases has far outpaced AMD's recently.
This is all in regards to the PC space. I don't know if AMD is doing anything special for the PS4 (I haven't read any of the news or announcements about it), but my first guess is that they're using their existing "8-core" design, which is basically 4 instances of a core that features 2 distinct execution engines. I could be wrong, though...
And so ends your microprocessor architecture lesson for the day (sorry, I do this stuff for a living).