Ah, okay. Anil, it looks like there might be some mixup in your install. The string "caml_hash_mix_double" only appears ocaml-4.00, but nowhere in ocaml-3.12. Also, I was confused. It turned out I was testing with 3.12.1 on Mac OS after all, which is why I didn't catch that unused value warning. I'll install and test with 4.00 on a Mac tomorrow.
A few of your sexplib changes didn't make it into this release, and I'll take a look at your patch queue to see what else we're missing. But I've been building and testing on Mac OS X. I'll see if I can recreate Anil's missing symbol problem (I haven't been testing with 3.12.1)