My desktop icons' layout (the last one I had when logging out from xfce) gets messed up on xfce's start when I use a theme of mine which has those lines in its "gtk-2.0/gtkrc" file:

----
style "xfdesktop-icon-view" {
 XfdesktopIconView::cell-text-width-proportion = 1.5
}
widget_class "*XfdesktopIconView*" style "xfdesktop-icon-view"
----

If I remove them I have no problems.
Also, I have no problems if I use any other "XfdesktopIconView property"; and, strangely enough, I have no problems if I put those same lines into my "~/.gtkrc-2.0", keeping them into theme's "gtk-2.0/gtkrc" (so that's the "workaround" I'm using).

It seems that xfdesktop, when these lines are only in my theme's "gtk-2.0/gtkrc", doesn't correctly detect "desktop size": until I do a "xfdesktop --reload", I can't drag-and-drop any icon outside the small "box" xfdesktop "sees" as "desktop size".

Perhaps this can be related to...
http://mail.xfce.org/pipermail/xfce4-commits/2012-March/029905.html
(although my xorg.conf uses only one monitor, I have 2 of them attached, one "on" and one "off", and I see them both in xfce's screen setup).