Make sure you have the package genisoimage installed on your system. If your running Ubuntu Feisty like myself, you should be able to get if from the universal repository.
billy@linux:~/Microsoft/W2K3ENT$ geteltorito win2k3ent.iso > win2k3ent.img
Booting catalog starts at sector: 20
Manufacturer of CD: Arnes Boot Record
Image architecture: x86
Boot media type is: no emulation
El Torito image starts at sector 541 and has 4 sector(s) of 512 Bytes
Image has been written to stdout ….
As you can see the image file is now present in the same directory as the original image file.
billy@linux:~/Microsoft/W2K3ENT$ ls -l
-rw-r–r– 1 billy billy 2048 2007-06-07 11:29 win2k3ent.img
-rw-r–r– 1 billy billy 617533440 2007-06-06 22:33 win2k3ent.iso
-rw-r–r– 1 billy billy 29 2007-06-06 22:33 win2k3ent_serial.txt