Well, it's fun to join the old discussion in the new times.  The fact  
that computers go multicore at a greater scale makes it recurrent.

Erlang makes concurrency easy due to purity, and OCaml is famous for  
being eclectic.  Why not embrace Erlang's model by imposing  
limitations on what can be in threads -- keeping them pure?  Erlang  
model works and attracts people to functional programming in general.   
Even if some other model of concurrency prevails, it is interesting  
and useful to interop with Erlang easily.   Here's what Erlang folks  
have started:

http://code.google.com/p/erlocaml/

Doing this properly can solve a lot of needs out there, and bring lots  
of debugged, proven, high-quality concurrent server and communication  
code within reach.

Cheers,
Alexy