making an xubuntu.img on a flash drive on mac http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx

prep_usb.sh

# 
curl -o xubuntu.iso http://mirror.anl.gov/pub/ubuntu-iso/CDs-Xubuntu/12.10/release/xubuntu-12.10-desktop-amd64.iso 

# iso to img
$ sudo hdiutil convert -format UDRW -o ./xubuntu.img ./xubuntu.iso
Password:
Reading Driver Descriptor Map (DDM : 0)…
Reading Xubuntu 12.10 amd64              (Apple_ISO : 1)…
Reading Apple (Apple_partition_map : 2)…
Reading Xubuntu 12.10 amd64              (Apple_ISO : 3)…
.............................................................................................................................................................................
Reading EFI (Apple_HFS : 4)…
..............................................................................................................................................................................
Reading Xubuntu 12.10 amd64              (Apple_ISO : 5)…
................................................................................................................................................................................
Elapsed Time:  4.667s
Speed: 148.7Mbytes/sec
Savings: 0.0%
created: /Users/mmcdaris/Downloads/xubuntu.img.dmg


$ diskutil list
➜  Downloads diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *121.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            120.5 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *61.8 GB    disk1
   1:             Windows_FAT_32 USB DISK                61.8 GB    disk1s1
➜  Downloads

# remove the dmg extension
$ mv xubuntu.img.dmg xubuntu.img 

$ diskutil unmountdisk /dev/disk1; sudo dd if=xubuntu.img of=/dev/rdisk1 bs=1m
# Just wait for it to finish...
Unmount of all volumes on disk1 was successful
Password:
694+0 records in
694+0 records out
727711744 bytes transferred in 168.067158 secs (4329887 bytes/sec)

# now just eject the disk
$ diskutil eject /dev/disk1
Disk /dev/disk1 ejected