Starting with WPS 6.1.5 the Portal ships Dojo 1.3.2 version and as a result the CSA theme works in both Internet Explorer 8.0 and Mozilla Firefox 3.5
As you can see i am using Firefox 3.5.5 (3.6 beta) and you can see that i am using a CSA theme and portal is making call ATOM service to get parts of the theme.
This is my screen shot of Client side aggregation theme being used in Internet Explorer 8 and as you can see i dont have to switch to the compatibility mode (IE 8 has concept of compatibility mode, if the website that your accessing is not compatible with IE 8 then you can switch into compatibility mode and IE 8 will behave like IE7)