Paul_o_b - 8:10 pm on Sep 27, 2011 (gmt 0)
Come to think of it... IE6 and IE7 always have a vertical scroll bar present on the browser window (greyed out when not required)!? And this certainly seems to be the case in IE8 under "IE7 Browser Mode" and in IETester (IE6 and IE7). This probably explains why trying to set overflow-y:scroll on the body produces an additional scroll bar in these browsers!
The problem actually seems to be that IE6 and 7 apply overflow-y:scroll by default to the html element thus when you also set scrollbars on the body you end up with two sets of scrollbars.
If you set the html element to overflow:auto then the default faded scrollbar disappears much like other browsers.
The specs basically say that overflow applied to the body element should be propagated to the viewport.
11.1 Overflow and clipping
UAs must apply the 'overflow' property set on the root element to the viewport. When the root element is an HTML "HTML" element or an XHTML "html" element, and that element has an HTML "BODY" element or an XHTML "body" element as a child, user agents must instead apply the 'overflow' property from the first such child element to the viewport, if the value on the root element is 'visible'. The 'visible' value when used for the viewport must be interpreted as 'auto'. The element from which the value is propagated must have a used value for 'overflow' of 'visible'.
It seems that IE6 and 7 have misinterpreted the above which is not surprising as its not entirely cut and dry.