I wasted a significant amount of time over the past couple of days trying to persuade PhotoGroup to go full-screen using _NET_WM_STATE_FULLSCREEN rather than by asking GDK for the screen size and setting the window size to be the same. (It turned out the problem had been that the window type I'd previously set, to get rid of the window borders, was conflicting with the full-screen setting and preventing it taking effect, rather than that I was doing anything wrong in my xlib code.)
You might ask why I bothered, given that the new method is more complicated and won't work in some window managers? Well, I was rather fed up that when I tried to look at my own photographs the pictures were getting displayed using both monitors, and therefore had a huge gap exactly in the middle....
(The new method does the right thing for me, because my window manager, Sawfish, knows that the single Xinerama screen is really two bits of hardware.)
More GPE tidying up for the forthcoming Familiar release. At some point today I was discussing how we should let the user set the screen brightness/rotation - we had a GPE-Screen for this, but it assumed the user was logged into X as root, something which unfortunately used to be true in Familiar, but which we're of course heavily discouraging. Then I remembered that a while ago I had written a little GTK app for this. Anyway, pb pointed out the obvious, so I checked the sources into CVS, tidied things up a bit, and made the minor changes needed so it would link against libgpewidget. Now we have a screen settings program, at considerably less effort than it would have taken to fix GPE-Screen.