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:
http://www.showsec.co.uk/admin.php?S=24384227da408225ee16a159edfba4966ac36622&D=cp&C=javascript&M=load&file=css&theme=default&v=1369159495
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!