IE7 is now CSS 2.1 compliant, well nearly.
We have been struggling with Rainbow themes and CSS compliance for quite a while and it appears there is a way forward. I happened to find this article whilst looking to fix the new IE7 CSS problems on the Rainbow website
http://www.sitepoint.com/blogs/2005/10/13/microsoft-says-de-hack-your-css/
I'm going to look to see if we can put a separate CSS file for IE5, IE6 and IE7. Then changes that are needed can be separated logically. This is certainly something we need to do to fix the Rainbow theme issues in the older versions of Rainbow.
I do not know if the Microsoft themes and skins will be similarly affected in the new Rainbow 2, but I will look into this soon.
I recommend that everyone use IE7 to view the website as it is the closest Microsoft has come to producing something that is CSS 2.1 compliant.
regards, Mark