To work with disk images, you use the hdiutil command. (See Apple's online manpage.)

hdiutil supports the following filesystem types:

Create a DOS disk image:

        hdiutil create -fs ms-dos -sectors 2880 floppy

(2880 sectors = 1.44mb floppy disk image)

If the image already exists, you must pass the -ov option or hdiutil will fail.

To create no apple partition image, use:

        -layout NONE

To create an image with an apple partition layout, use:

        -layout SPUD

I dont know if images created here are straight binary images that can be plugged into Bochs/VMWare etc...

        hdiutil create -fs MS-DOS -sectors 2880 floppy

This will output floppy.dmg. Note that the MS-DOS argument is case sensitive.

        hdid -nomount ./floppy.dmg

This will output the name of the device file created.

        mkdir /Volumes/mnt
        mount -t msdos /dev/disk2 /Volumes/mnt

Use the name of the device file output in the previous step.