I don't think multicore will be touted as a reason to choose OCaml over Haskell, but it will remove one of the reasons to avoid OCaml. Subtle but important difference :)
It won't though. Because it'll still be a decade behind haskell. People want more than just "ok, you can use multiple OS threads now". They want a concurrent IO manager, painless and seemless multiplexing of low cost green threads over OS threads, and high level concurrency and parallelism constructs. It is going to take a long time for ocaml to catch up to haskell in this department, and who knows what haskell is going to be offering by then.