I am using EE v2.5.5 and the control panel doesn't work properly in IE 9 and 10. The issue seems to be that EE doesn't know you are logged in when a script is requested and so instead of returning the JS you just get the login screen instead. SO the URL below for example:
Just returns the login page even though I am logged in
In fact if I login to the control panel in IE 9 or 10 and then open a new tab and go to the control panel URL I just get the login screen again. The browser has forgotten I am logged in already!