Probably something really silly I'm missing, but...

If I manually add an icon to /usr/share/icons/hicolor/scalable/hildon/myapp.png and then write an application.desktop metafile in /usr/share/applications/hildon and point Icon=myapp the icon does not work but instead uses a default Maemo 4-square icon.

However, I recently installed some other application from an actual proper Maemo install, and after that application installed properly, the icon I had added started working.

So the installer must obviously call some kind of updater that refreshes an icon cache or database to make the icon visible and usable to the task launcher.

Can someone explain where this takes place?

In case anyone is trying to figure this out, there is indeed an icon cache updater.

If you've changed icons around in /usr/share/icons/hicolor you can force the cache to update by doing:

gtk-update-icon-cache -f /usr/share/icons/hicolor