Long time since I posted before. I take back the warnings about making a jffs2 image with mkfs.jffs2 and reinstalling it with 0xFFFF. I have done it several times now, with a fully equipped rootfs, and it worked like a dream.

Same thing if you have a customized initfs with bootmenu that you will want to reinstall after a full reflash. (In that case you would make a jffs2 image from /dev/mtdblock3 instead of /dev/mtdblock4. You would also call your image something different from rootfs.jffs2 - say initfsCUST.jffs2 .)