Beside IE7 support to transparent PNG and RSS, IE7 has gone very near to CSS2.1 specs, they still miss some points but it's way better than before, the IE team has announced the list of CSS changes on there blog post titled Details on our CSS changes for IE7, it's essential for every web designer who care about web standards to read.
I'm really glad that the end is near for IE6 especially after Microsoft putted IE7 as a high priority update on windows Automatic Update.