canon powershot S30 - circular shadow?

I recently noticed a weird shadowy thing on some of my photos taken with my Canon Powershot S30. It only shows up when I am zoomed in (optical zoom) and it's more noticable if there is a solid colored background (sky, wall, etc) It is perfectly rounded, so I'm almost positive that it's not a scratch, and I've cleaned the outer part of the lens, viewfinders, etc carefully to make sure it's not just dirty. Here is an un-edited example of what I'm seeing:

http://muted.com/pictures/test.jpg

The size and focus of the shadow changes when I adjust the zoom in and out.. when it's completely out (wide-angle) it's almost invisible, but as I zoom in, it gets more and more noticable, darker and clearer.

The camera is a year and a half old, so the warranty is over, unfortunately. I'm just wondering if anyone else has had a similar problem with this or any other camera, what might be wrong, and how I might fix it. Thanks for your help! :)

Possibly a hair or fiber trapped inside the lens. Look at the lens mechanism carefully, zoom in and out. You might be able to see it. Ifnot, time to send it in. If it's just a cleaning then it shouldn't be too much.

Thanks! I did what you suggested, but I don't see anything inside the lens when I zoom in/out. Maybe I'll check about getting it cleaned- any advice as far as that goes? Would a local camera repair place be okay, or should I try somewhere else? Or ship it to the manufacturer? :confused: