the fix is to remove the users kde cache directory (when not in kde)
rm -rf /var/tmp/kdecache-`whoami`
I only had this problem on the first user to run x (startx) which was root. Logging in as subsequent users worked without the icon problem, including uninstalling the app, logging in as another user, installing as root while in X.
I'll see if the problem is the same for slackware 13.37 64bit. I'll post the results back here, and on the mailing list for info.
It seems to be only affecting the 32x32 png icon. Other sizes are fine in dolphin, and are not used for kickoff (normal view)
As to why the problem occurs, it as been suggested http://forum.kde.org/viewtopic.php?f=67&t=39274&p=223326#p223321
that the icons be installed before the desktop file. however, doing that on a slackbuild, which installs alphabetically so /usr/share/applications/project.desktop is always installed before /usr/share/icons/[16x16,32x32,48x48x64x64]/apps/project.png would prove a bit of a problem. Shy of putting the .desktop file in install, and then copying it to /usr/share/applications in the doinst which is a bit messy, as i'm not sure how it would get cleaned up on uninstall
Well I've tested with clean slackware13.37 64bit install and no problems with icons on kickoff or dolphin. Either I was unlucky in the timing of icon scanning for kde, or the cache got messed when I had to kill X the first time (ctl-alt-backsp) as the nouveau drivers didn't like my hardware and I had to install the proprietary nvidia ones.