I'm still unconvinced that the current optional RTS support for mixed green/native threads is the right way to go. It looks to me like a workaround for poor OS support for really lightweight threads.

It is a workaround for the lack of truly lightweight threads at the OS level. But I don't expect that situation to change, even with the recent improvements on the Linux front. The new RedHat Linux threads folk claim to be able to create 10^5 threads in a couple of seconds, whereas we can create 10^6 threads in about that time (on an old 500MHz machine, too). Cheers, Simon