I'm surprised to hear that gnome-core requires gnash. It's certainly not the case on Lenny:
I it was introduced into squeeze yesterday (source package meta-gnome2 2.28+6 transitioned to squeeze on 2010-03-16). The reason seems to be that swfdec was removed. The changelog[0] for meta-gnome2 says (among other things):
* Drop swfdec-gnome. * Make gnome depend on mozilla-plugin-gnash so that there’s at least something for Flash websites.[0]: <http://packages.debian.org/changelogs/pool/main/m/meta-gnome2/current/changelog>
/ johan