Here's one example of why it's a good idea to subscribe to the gentoo-dev and gentoo-user lists:
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From: "Justin T. " <justint@...>
To: gentoo-user@g.o,
	Prashanth Aditya Susarla <aditya@...>
Subject: Re: [gentoo-user] Testing opengl
Date: Fri, 19 Apr 2002 15:31:20 +0200
Reply-To: gentoo-user@g.o

The problem is actually with the ebuild for kdeartwork (from what I have 
seen):

    use opengl && myconf="$myconf --with-gl" || myconf="$myconf --without-gl" 
    myconf="$myconf --without-gl"

This was taken from the .ebuild file for kdeartwork.  It is changing the use 
to what the person requires, but then goes through and cancels it out again.

I went through and commented out the second line:

 #myconf="$myconf --without-gl"

And then recompiled kdeartwork... OpenGL screensavers now work perfectly for 
me.  Of course this is a change in the file that will disappear with the next 
"emerge rsync", so be aware if you recompile that you check this.

Not sure exactly why they have this blocked out (could be that it causes 
problems with certain setups), however it works perfectly here.  (Athlon 
1200, NVidia TNT2, and everything compiled with the highest optimizing...)