I would like to draw the attention of theme developers to a change that took place recently in cvs: the "admin_tree" icons are now themeable, which is to say that open.gif, closed.gif and item.gif have been moved to /pix/i/.
This also means that they must be copied into any theme that duplicates this folder. Otherwise they won't show up in the site.
For a site admin, this means copying the three files from folder /pix/i/ to folder /themes/yourtheme/pix/i
This change has been done in version 1.8 and 1.9. Chameleon was the only core theme that needed to be updated: if you are using a core theme, you don't need to change anything.