The standard CSS3 rotate should work in IE9, but I believe that you need to provide it with a provider prefix, for example:
-ms-transform: rotate(10deg);
It may not work in beta; if not, try downloading the current preview version (preview 7), which is a later beta version. I don’t have a beta version for testing, so I can’t confirm whether it was in this version or not. The final release is definitely intended to support it.
I can also confirm that the IE-specific filter property has been reset in IE9.
[Change]
People asked for additional documentation. As the saying goes, this is pretty limited, but I found this page: http://css3please.com/ , which is useful for testing various CSS3 features in all browsers.
But testing the rotation function on this page in IE9 preview caused it to crash quite spectacularly.
However, I did some independent tests using -ms-transform:rotate() in IE9 on my test pages, and it works fine. Therefore, I came to the conclusion that this function is implemented, but there are some errors, possibly related to its dynamic configuration.
Another useful checkpoint for which functions are implemented in which browsers - www.canIuse.com - see http://caniuse.com/#search=rotation
[EDIT]
The revival of this old answer is because I recently learned about a hack called CSS Sandpaper , which is relevant to the question and can make the task easier.
Hack implements standard CSS transform support for older versions of IE. So now you can add the following to your CSS:
-sand-transform: rotate(10deg);
... and work in IE 6/7/8 without using filter syntax. (of course, it still uses the filter syntax behind the scenes, but it greatly simplifies management because it uses the same syntax for other browsers)