I’m integrating Game Center into my app using the Prime31 plugin, which is great. I posted an achievement in the sandbox then went to check if it appeared on the Game Center. It had, but with a graphical glitch on the icon, that looks to be standard.
The achievement icons are shown within a white circular border. But the edges of the image are clipped with a white area beyond. I found a website that showed the same thing, the link is below. Look at the Safety Card icon on the second image down. See how the white circle has some straight inner bits? Looks wrong doesn’t it?
http://52tiger.net/some-game-center-screenshots/
To get around this, I am adding a circle to my icon images, inset from the edge and colouring everything white out from it. It looks much better with a thicker, but consistent white icon border. My tip is to do the same.
Now back to uploading new icons for my 39 achievements, each with 7 language options. It’s taking a while.
What kind of files are you using for the icons?
I don’t know anything about Prime31, but looking at their website, I should take a close look 
512x512 png
Here is a link to the Prime31 thread for the Game Center plugin. You will find more threads for the other plugins. Excellent customer support.
http://forum.unity3d.com/threads/59551-GameCenter-Plugin-Live!-Leaderboards-and-Achievements!
Did you resolve the issue?
What program are you using to create the PNG and what settings do you use for the PNG export?
I just did as I said in the tip, my images are white outside of a circle that doesn’t reach the edge of the image. That way the unwanted extra bits of white blend in with my image.
Any art package will do. The process doesn’t seem to worry if there is transparency or not. I use Adobe Illustrator and export using the Export to Web option as it gives good quality and small file sizes for quicker uploading. I have Clip to Artboard set as I have the artboard set to 512 pixels by 512, which means I get the perfect size image easily.
I also do the same artboard process when creating any images I want to be a perfect size. Great for the power of 2 images that Unity likes.
I just finished a game for iPad and noticed the problem no longer exists in ios 4.2
512x512 squares are fine now…at least for iPad.
I assume they fixed it for both.