Parallelism in Lisp

Maybe not as hot a topic in computer architecture as it used to be, but still of considerable interest, is parallelism. How do you make a faster computer? Just strap 20 or 200 or 2000 processors together? As we have learned, the architectural and hardware difficulties are immense (How do you connect them? A shared bus? A network? Is there a single system clock or many clocks?), and after these have been solved there remains the matter of programming.