The N900 comes with busybox. Busybox doesn't use the GNU coreutils utilities/commands instead it has watered downed versions with less features. BASH vs ASH is mostly syntax related.

Install this: http://maemo.org/packages/view/coreutils-gnu/
Or simply compile coreutils in scratchbox yourself and copy over the binary find command.
Woops find is not part of coreutils but instead findutils.