multi-threading has yet to mature to the level of using 8 cores simultaneously..however the application itself can be told to use those cores, but one has to properly align the respective tasks to handle their cores. If the application isn't multi-threaded to start out with, or limited to 2 cores only, the result will not be that good if it were 8-core native (2 x proccesors, with 4 x cores each). AMD will also be coming out with their own 8-core arch soon.
Scientific and Programming industries have been using multi-processing for a long time. Mid-range workstations and mainstream machines only got into the game late, and it will take time. For video, if you look at SGI, their older machines had upto 4 MIPS processors, at a time when multi-processing was unheard of.
Now the technology lets more than one core fit inside one processor die, thus it will be cheaper than 2 processors (but it will not provide equivalent speeds in terms of clock-cycle, MHz/GHz). Also, coding software to intelligently take advantage of those cores is harder than telling it to take advantage of 2 physical processors, and that's why we're only progressing at 2 cores for the time being. If you think of Maya, 3DSM, Blender, they are capable of handling multiple processors, but limited when it comes to cores (although they have intelligent code to sort out more than 4 cores with 2-4 processors and will probably work, but not with full potential). For audio applications, same thing applies.
If you want to get this MAC, you are future-proof, but you will have a few barriers with software starting this early. For mainstream workstations like this, you have choice for upto 2 processor slots. So quad-core processor x 2 = 8 core. So why not just get a single quad-core?