Thanks, Ash. The switches you pointed out fixed the problem. I didn't use the exact commands you did, but modified the script on gumstix.org:

flash_eraseall /dev/mtd4
mkfs.jffs2 -n -e 128 -o /dev/mtdblock4
echo "Mounting rootfs partition ..."
mkdir -p /media/mtdblock4
mount -t jffs2 /dev/mtdblock4 /media/mtdblock4
cd /media/mtdblock4
tar xjf $dir/$rootfs
sync