There is no such thing as a BIN file.
The tablet has three user flashable parts without risk of bricking it beyond the user hability to fix it by reflashing: the linux kernel, initfs and rootfs. The flashable linux kernel image is a standard zImage (the kernel in compressed form). The initfs is a little ramdisk image that is loaded by the kernel at boot time to start the system, and the rootfs is the real system.
There is also a FIASCO image. That is a file which have the propietary and undocumented system parts used to boot the tablet, the kernel, initfs and rootfs, but FIASCO is a propietary undocumented file format used by Nokia to distribute the whole system. You don't need (or even can) to build a FIASCO image to backup your tablet.

You can make a simple file backup of the whole rootfs in a tar.gz archive. That backup can be used to restore the system in a linux ext2/ext3 partition in one of the mmc cards by decompressing it there, and you can configure the tablet to boot from there by using flasher3 ("flasher-3.0 --set-root-device mmc").

If you want a flashable image to write in the system flash, you have to build it using mkfs.jffs2 as explained in the mtd-tools thread, or get it with 0xFFFF. That image is a file containing a jffs2 filesystem. Usually you only need rootfs, unless you also want to play games with initfs (boot menu and such).

Really, the best idea is to boot from mmc, then you can play and do whatever you want without risk of bricking the tablet and have to reflash it. If you break the system in mmc, you can restore it in a PC with a card reader o you can boot the tablet from flash ("flasher-3.0 --set-root-device flash") and restore the rootfs in mmc from the tablet.
I even have a mmc with the original diablo system and another mmc with debian, and boot from either by swapping cards.