I dunno why you would want to model a computing system after the human brain... Yea, sure, the brain is powerful, but it is much too flawed for computing environments, we can't precisely do anything more than once, for example. The only other reason why you would want to build one of these things is for social type crap, such as a robotic woman that doesn't scream at you for leaving the toilet seat up... for example >_>
Getting the hardware built is just a matter of time, figuring out how to best write software for it will be the real difficulty. Similar to programming using DNA (which is horrendously complex/difficult on all but the simplest strands), nature has had billions of years of trial and error to come out with complex organisms that function reasonably well (the human brain is amazing but certainly not infallible). We can try to modify existing life, but coming up with our own from scratch is far beyond current capabilities.