As far as win32/un*x comparisions go, I've seen benchmarks according to which linux process were faster then win32 threads.

With threads==lighter processes on un*x you get maybe 0.5% improvement, while programming/debugging becomes way harder (maybe not for java addepts, who are raised with 'threads are god/good' mentality).

Thus, I think, efficency is not a good reason for such strange feature, locality of data, maybe, wider availability of Niagara-like processors, maybe.

And with perl threading stabilising ( for me one such sign is new debian stable shipping threaded perl by default ), we should be ready for technologies like SMT.