I thought I should mention this to keep people informed about
large projects done in OCaml.

Micromagnetism is a rather hot topic these days. There are
a number of packages around that allow one to do physical
simulations of sub-micron scale magnetic systems, but there
is one around (ours) which is written mostly in ocaml.
(The user interface is python, though.)

The rationale for using ocaml was that we needed both speed
and enough flexibility to do symbolic manipulations.
Actually, this micromagnetism package is just a library on
top of a much more generic multiphysics simulation system
that was designed to deal with a very broad class of
generalized reaction-diffusion systems, whose ultimate goal
is to automatically compile an abstract problem specification
(containing differential operators and equations of motion
in symbolic form) down to some numerical finite element code.

The first beta-release (still a bit wobbly) is at:

http://nmag.soton.ac.uk

-- 
best regards,
Thomas Fischbacher
t.fischbacher@soton.ac.uk