When a user presses a launcher button, it's not obvious that anything is launching. This is especially important if it takes a while for a screen to show. One reviewer's comment on Unity (http://www.david-chung.com/dave_blog/?p=105) is that: "One thing I loathe is that the launcher items doesn’t show any indication of me launching something. That is, if I click or activate a launcher, it does nothing. I expect the launcher to animate, blink, or something to tell me that I launched something." The approach used by GNOME 2's gnome-panel's is that the icon "grows to about 3x and is translucent" (when there's a compositor): http://ubuntuforums.org/showthread.php?t=1531843 It need not be that particular effect, but an indicator that something is happening would be good. If it's hard on the processor, make it optional (say, as a panel option).
You see a button effect if you click the item, like all gtk buttons. If the program supports startup notification also a wait-cursor is shown (freedesktop spec). The panel itself has no idea when a program is finished launching, although that is possible with startup-notification, I don't see the need to add something like that.