That would be nice. Sadly, the default virtual keyboard is closed source and the example keyboard sucks. It could still be possible, but I don't yet know how. It will make your applications crash and might brick your n900, but you can place it in /usr/lib/hildon-input-method/, run hildon-im-recache as root and then set the gconf key /apps/osso/input-method/default-settings/finger to "him_cellwriter". Soon. I just submitted a request to the garage for a git repo.