If soft-switching is portable (i.e. pure C, no assembly), and is exposed as a library module (so that Jython et al can avoid supporting it), then perhaps a PEP for adding that functionality to mainstream Python would be meaningful.
Anything that can be supported by a pure extension module is fair game IMO. But it was my understanding that Stackless requires changes to the VM. And I definitely don't want people to get into a habit of writing code that relies on deep recursion that is only supported by Stackless, only to find that it doesn't work in Jython etc.
If that gets in, then the other 5% can always sneak in later via feature creep. ;) Or, more importantly (if I understand correctly), it could be separately distributed as an add-on for platforms that can support it.
Hey, that's what Stackless already does. Why is that approach suddenly not good enough any more?
--Guido van Rossum (home page: http://www.python.org/~guido/)