There are certain themes that have invisible icons that disappear until your roll over them. For these check out the Media section. The "zen" style themes only have a row of icons at either the side or bottom of the screen. These are nice for being able to see most of a wallpaper.
To hide individual icons you can highlight it, press the menu button and then select "hide icon". They will all show again if you select "show all." Hidden icon status can be edited by this same process.