How often do you think it can be expected that your program will have the whole of the computer to (ab)use? How often do you think the computer doesn't have several other tasks to run?

IMnsHO for most applications, one core will be plenty and the multiple cores will be used by multiple processes. For most of the rest, splitting the task into a few parts and running several instances of the program will be both good enough and easiest. So no, I do not think "multicore" programming is something most developers will need to care about in the near to mid future.

There is one big problem with the (even partially) automatic concurrency. If you split the task into too many too small pieces then you waste more than you gain. A perfect example would be the paralelization of something like @result = map {$_*2} @source;. The task you need to do with the items is simply too simple and inexpensive. Even if you split the (huge) @source into just a few parts and parallelized those, the overhead would slam any gain. But once the chunks get sufficiently big, they usually also get too complex to parallelize automatically. So I'm a little skeptic.

P.S.: I believe the concurrent stuff has been taken out of Clean. It did use to be called "Concurrent Clean" originally, but the concurrent evaluation annotations had been removed in version 2.0. Not sure what's the current status of things.