If you're running XP, right-click on the taskbar, Properties, Customize.
There you can select if something is showing all the time, only when it's active, or never. An an arrow between the system tray will always point to the left if there are more icons not displayed, but otherwise, I don't know of a way to make them disappear.
lol me i should have done more searchig for it i found a app specially made for it at http://www.xdesksoftware.com/systrayx.html
hafta pay for it though(yea right...astalavista)