Quick'n'Dirty HowTo:

 0) patch and build qemu :)

 1) create the nand image:

	qemu-img create -f qcow2 nand.img 640M

 2) start qemu with the devel images, like this

	qemu -nographic -snapshot -hda olpc_ext3.img \
		-hdc olpc_jffs.img -nand nand.img \
		-kernel olpc-kernel \
		-append "ro root=/dev/hda1 init=/bin/bash"

 3) verify that the nand is there
	
	# mount -n /proc/
	# cat /proc/mtd

	dev:    size   erasesize  name
	mtd0: 20000000 00020000 "NAND 512MiB 3,3V 8-bit"

 4) initialize the flash memory

	# mount -n -o remount,rw /
	# mknod /dev/mtd0 c 90 0
	# mknod /dev/mtdblock0 b 31 0
	# /usr/sbin/flash_eraseall -j /dev/mtd0

    you might want to write back the nand at this point
    (with CTRL-a n) and copy it somewhere, for later

 5) copy the jffs2 image to the flash memory

	# mknod /dev/hdc b 22 0
	# /usr/sbin/nandwrite -p /dev/mtd0 /dev/hdc

    again, writing back and saving the image might be
    a good idea, if you want to go back to this point

 6) mount and/or use the jffs2 image ...

	# mount -t jffs2 mtd0 /mnt
    or
	# mount -t jffs2 /dev/mtdblock0 /mnt

best,
Herbert