added more description as below:
Just in case, the following is what I did/saw :
1. I already upgraded ios 5.1.1 and there's no problem about Game Center before restoreing ios.
2. I restored ios(iphone 4s) via iTunes ( ver. 10.6.1.7) on windows pc
3. After finishing the restore (I tried it with both backup and as-new-setup), launch Game Center app and log-in as my Game Center ID
4. Open "Friends" tab and see the friend list on the left of the screen
5. See description below all friends' name shows "Never Played"
I tried log-out and re-login Game Center, but this wasn't fixed.
I feel very sad to see my Game Center ID looks broken.
But I'm also guessing this issue comes from Game Center's server side, because this worked fine before restoring ios.