I just checked out one of my school's spiff-tastic gl2's a few weeks ago to shoot a sporting event. When I dumped it to my computer, i noticed that the corners of the image were darker than a nice circle in the center. Is this because of my lens shield? I was shooting in the evening at my friend's tennis court with the overhead halogen lights turned on. Its not limited to that camera, because I was also using a GL1, and it had the very same problem. Any suggestions?

This is a normal attribute of the lens' design, known as edge fall-off. It will be more apparent under certain shooting conditions. We have a very long thread on this topic dating back nearly two years. But that's the bottom line.

I noticed the same effect when reviewing shots taken at twilight on a lake shore recently. Now that I know it is a lens attribute, I'll make allowances for it in the future. (Will try to avoid shooting at full zoom in low light.)

Here's the thread Ken was referring to:
http://www.dvinfo.net/conf/showthread.php?s=&threadid=3430

I thought it was due to the iris opening too much. If you lower the shutter speed or film on manual this effect goes away.

That's correct.