Version:            (using Devel)
Installed from:    Compiled sources

In a default installation of the GIMP its toolbox windows are marked as utility windows. These are excluded from the alt-tab switch but not the task panel whhich sometimes (not always, this seems to depend on the speed of the machine or other tasks performed during application startup) the initial image window isn't picked up by the task panel. 
IMHO this inconsistent behaviour (alt-tab excluding the utility windows) is not warranted by the documentation at http://standards.freedesktop.org/wm-spec/wm-spec-latest.html where windows with the _NET_WM_WINDOW_TYPE_UTILITY hint are described as non transient windows (even if they may set the WM_TRANSIENT_FOR to identify their associated main window)...
Currently the only way to work with the GIMP is to change the toolbox hint to normal window.
In KDE3 kwin the utility windows and image windows are consistently picked up by both the task panel and the alt-tab window switcher.
For some more information as to how to reproduce this behaviour and to see it happening see bug 177025...