"if you use correctly use multithreading" Good luck doing that 100% of the time!
As for Erlang, I'm pretty sure it wasn't meant for numerical calculations, and since it's interpreter, it can't really be used for all real-time apps. Although it's process model is novel, the implementation might not be perfect for everything right now. Correct me if I'm wrong on this (which I might well be, since I have not spent too much time writing Erlang code.)
I think what the parent meant is that writing multithreaded programs easily and quickly (not to mention bug-free) is hard, hard enough to be called a problem.
Being able to program against multiple cores/processors/machines is one of the biggest "unsolved" problems facing us right now.
Intel could make itself a real brand boost by embracing external developers, especially if you plan on hacking C/C++.