Can't associate custom icons with desktop launchers

[Guy Rouillier <guyr-ml1 burntmail com>]

I installed monodevelop on Gentoo AMD64 2.6.11-6, and in so doing messed
up my desktop launchers.  The first problem was that they lost their
launcher behavior; they were shown with the .desktop extensions, and
Properties stated they were text files.  The following suggestion from
the Gentoo desktop environment forum fixed this (and monodevelop has
been fixed):

update-mime-database /usr/share/mime/

My remaining issue is that I cannot associate custom icons with existing
desktop launchers.  In the Properties dialog, I can click the "Select
custom icon" button and choose an icon, but it doesn't do anything; the
icon associated with the launcher doesn't change.  Strangely, the
"Remove custom icon" button becomes enabled; if I click it, nothing happens.

Some other observations:
(1) I can successfully specify a custom icon when initially creating a
new launcher;  having done that, I can't change it.
(2) I can successfully specify a custom icon for Home, Computer, and the
 partition icons, all of which are of type "folder".  I can also
successfully remove these custom icons.
(3) I can successfully specify a custom icon for "Start Here", but I
then cannot remove it.  "Start Here" is of type "desktop configuration
file", as are the launchers I create.  I don't understand why I can
specify a custom icon for "Start Here" but not for my own launchers.
(4) I manually edited a .desktop file and added an Icon= entry. When I
exited Gnome and did startx, the icon was changed, but my system locked
up; I had to power cycle.  Also, "Remove Custom Icon" is still grayed
out for that launcher.  So I'm guessing that custom icons are recorded
some place in addition to the .desktop file.

Finally my questions:

(1) Is there a simple way to get custom icons working again for desktop
launchers?
(2) If not, what component handles the custom icon selection and
removal?  I'm a programmer, and am able (and willing) to read through
source if someone can point me in the right direction.
(3)  Why are custom icons recorded in multiple places?  (.desktop file
and someplace else)

Thank you for your time, and I apologize for this long-winded message.

-- 
Guy Rouillier