DirectWrite uses a newer version of ClearType with subpixel positioning, which IE8 does not use. WPF also uses subpixel positioning, and it did have problems. And you know how they fixed it for .Net 4.0? By changing WPF to render like DirectWrite does. So
now, after the fix, WPF 4.0 text looks like DirectWrite does.
If they want to change this they'll have to update DirectWrite. It's beyond the scope of IE9.
GDI vs 2010 RTM comparison looks quite close (I'd need a color picker tool to say which is which). In comparison the IE8 vs IE9 preview looks nothing alike zoomed up. If the font and colors of the above image could be replicated in HTML and opened in IE8
and IE9 that might tell something. Or not. I'm not sure what it would tell. I'll try to figure it out.