Sometime back after downloading IE9 RC, I was experiencing problems with how GMail was being rendered. I had this problem with IE9 Beta too, but in that case the problem was solved up checking the "updated lists from Microsoft" under the Compatibility settings. In IE9 RC, this did not work. So, I filed a query on the MS forum, which you can see here
Today, I found that the problem had almost miraculously fixed itself.
Now, IE9 RC (and soon to be the final version!) looks good...damn good actually!