in reply to Parrot, threads & fears for the future.
High performance work tends to move parallel into clusters, and it has been known for years now that forked processes are easier to scale for clusters than multi-threaded processes. The reasons have nothing to do with Unix versus Windows, and everything to do with minimizing necessary interactions between parallel jobs. (And moreover, letting the OS know that interactions will be minimized.)
Naive parallelism also has a great future. Think "webserver". Performance is just fine with unthreaded code, it is easier to manage development, and you can get as much parallelism as you need by running lots of concurrent processes.
I'm sure that there is also a great future for threaded programs. However my current take is that that future will tend to be either very specialized code, or else for native GUI applications.
Now if Perl 6 wants to be all things to all people, it probably should include support for threaded programming. But even if it has great support for that (and Audrey's implementation may), I'd be willing to bet that the multi-threaded part of Perl 6 will not be used in most Perl.