I just compiled unison for the N900, basically untested, so no warranties and all... just a unison binary, text-ui only:

(Note the version number, you need a unison with 2.38.x on the other end!)

For the record, the compilation method:
I needed to compile ocaml directly from svn in the armel scratchbox since only the svn version can give you an ocamlopt native compiler for the armel architecture (EABI rather than the old arm). Then compiled unison from a several weeks old svn snapshot (since this is the version I also use on my other machines).

cheers