I'm on CS4 Illustrator and it seems when I resize while holding down the shift key, the object isn't staying proportioned. Also, when I resize an object by clicking on the width and height value in the menu bar, the object does not stay centered when it resizes.
Is a setting off?
Presuming you have ticked the Center Reeference Point in the Transform palette, both issues may be caused by corrupted preferences.
You may try to Ctrl/Cmd+Alt/Option+Shift during startup or to Move the folder to set things right.
I had this issue as well a whil back. Turns out it was because I had Hulu open. If you use Hulu, turn it off and your proportional resizing should work fine.