If Ie9 Is Not Working With Your Custom Visualforce Page

Adding some points

Firstly, you might be able to add the following meta tag to your pages:

<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8" />

This will tell IE9 that you would prefer it operate like IE8, and IE8 still works fine.  However, I believe this meta tag needs to be one of the first things declared in the page, and under some situations (like showHeader=true) you may not have control over that.  In that case, you can add the following to the Apex controller:

Apexpages.currentPage().getHeaders().put('X-UA-Compatible', 'IE=8');

Thanks