I'm really shy asking this question.. anyways.. It's something which has been always buggin' me - my system tray has the width of (X + 1) applets. In other words, if i have the RHN update applet, the system tray is wide twice as much as the applet, thus leaving an empty space which I define as a "waste of space" in my taskbar... Coincidentally I've seen the same thing in the machine of my friend, but strangely enough, we've seen another pc with RH.9 which had not this problem... Anyone can tell us why this behaviour of the system tray? Is it a bug?

i had that, i think it's the key thingy that appears to tell u that u've got root priveleges, and doesn't resize the tray when it goes, a way to tell is to resize the bar, then run redhat-config-xfree86 or some other configuration app then shut it down once the key appears, wait till the key goes, and see if it resizes the tray back.
If it does, shoot me, for i am wrong.
If it doesn't, there is absolutely nothing u can do about it as far as i know, except use GNOME, blackbox, fluxbox, afterstep, windowmaker, or some other window manager that doesn't include a stupid key gimmick.
Of course, being linux and open souce, there will be some way to disable it, i just don't know how.

nope it doesn't