Canon 70-200 2.8IS L - Problem with the lens, or with me?

Hi, all.

I have the above mentioned lens.

I am finding an odd 'halo' effect around high-contrast areas of images. The effect is quite noticeable at f/2.8 - but if I drop it down to f/4, it's not a problem.

The attached sample is a tiny crop of an image - 100% magnification of an area that shows the symptom.

The only real difference between the two images is the aperture.

I'd appreciate any comments: Is my copy bad? Should I send it in for servicing? Am I just not using it correctly??

Thanks!!!