Well, this probably won't be Mitchell's answer, but to me it's obvious that an uploaded human brain is less than 50 years away (if we avoid civilization-breaking catastrophes), and modifications and speedups will follow. That's a different path to AI than an engineered seed intelligence (and I think it reasonably likely that some other approach will succeed before uploading gets there), but it serves as an upper bound on how long I'd expect to wait for Strong AI.

There are many synergetic developments: Internet data centers as de facto supercomputers. New tools of intellectual collaboration spun off from the mass culture of Web 2.0. If you have an idea for a global cognitive architecture, those two developments make it easier than ever before to get the necessary computer time, and to gather the necessary army of coders, testers, and kibitzers.

Twenty years is a long time in AI. That's long enough for two more generations of researchers to give their all, take the field to new levels, and discover the next level of problems to overcome. Meanwhile, that same process is happening next door in molecular and cognitive neuroscience, and in a world which eagerly grabs and makes use of every little advance in machine anthropomorphism, and in which every little fact about life already has its digital incarnation. The hardware is already there for AI, the structure and function of the human brain is being mapped at ever finer resolution, and we have a culture which knows how to turn ideas into code. Eventually it will come together.