I'm looking at /usr/share/icons/DMZ-White/cursors, and there is:

$ tree -s /usr/share/icons/DMZ-White/
/usr/share/icons/DMZ-White/
+-- [       4096]  cursors
¦   +-- [         14]  00008160000006810000408080010102 -> v_double_arrow
...
¦   +-- [          5]  9d800788f1b08800ae810202380a0822 -> hand2
¦   +-- [          8]  arrow -> left_ptr
¦   +-- [      15776]  bd_double_arrow
¦   +-- [      15776]  bottom_left_corner
¦   +-- [      15776]  bottom_right_corner
¦   +-- [      15776]  bottom_side
...

... a bunch of files without extension, that GIMP cannot open.

Is there an editor where these files can be opened - or at least a converter to something like .png? I can note that ImageMagick display also failed to open these files...

Found also Gursor Maker - Cursor Editor for X11/GTK+; got the CVS code from SourceForge - it still uses Numeric (the old name of numpy), so to run it, you'll have to do:

#from Numeric import *
from numpy import *

... in xcurio.py, curxp.py, gimp.py, colorfunc.py - and comment the #from xml.dom.ext.reader import Sax2 in lsproj.py. With that, I got it running 11.04:

... but cannot get any files to open? So I thought I should grep for paths, nothing much came up - and when I looked into cursordefs.py, I simply had to paste this:

CURSOR_ICON = gtk.gdk.pixbuf_new_from_xpm_data([
        "10 16 3 1",
        "       c None",
        ".      c #000000",
        "+      c #FFFFFF",
        "..        ",
        ".+.       ",
        ".++.      ",
        ".+++.     ",
        ".++++.    ",
        ".+++++.   ",
        ".++++++.  ",
        ".+++++++. ",
        ".++++++++.",
        ".+++++....",
        ".++.++.   ",
        ".+. .++.  ",
        "..  .++.  ",
        "     .++. ",
        "     .++. ",
        "      ..  "])

Heh :) In any case, doesn't look like it will be much usable on newer Ubuntus, unfortunately...

Just tested XMC plugin as well - on 11.04, has to be built from source (from the link in the accepted answer); the requirements on my system resolved to:

sudo apt-get install libgimp2.0-dev libglib2.0-0-dbg libglib2.0-0-refdbg libglib2.0-cil-dev libgtk2.0-0-dbg libgtk2.0-cil-dev

... after that, the configure/make procedure in the INSTALL file works. Note that this plugin is a bit "sneaky":

... that is, you should use "All files" (as there are no extensions); cursor previews at first will not be rendered. Then open one cursor file; after it has been opened, then there is a preview in the File/Open dialog; but other than that, it works fine...