The way I understand it, and the only way I have done it, the "panel item background" will be the image used for any mouseover on the start panel, left and right sides. This image won't work when the mouse is over the "all programs" section, which we can only change the color of. If you have seen skins that have a mouseover for the "all programs" section, post a link to them, cause I have yet to see one, and would love to know how to do it if possible. If you are talking about the menu that pops up after "all programs" is highlighted, look for "menu", and "menu item", right under "bottom bar" in the treeview in Skinstudio, but I doubt that's what you mean.

Anyone else feel free to correct me if I'm wrong, cause it wouldn't surprise me much if I was!