Standard CSS3 rotate should work in IE9, but I believe you need to give it a vendor prefix, like so:
It is possible that it may not work in the beta version; if not, try downloading the current preview version (preview 7), which is a later revision that the beta. I don't have the beta version to test against, so I can't confirm whether it was in that version or not. The final release version is definitely slated to support it.
I can also confirm that the IE-specific filter property has been dropped in IE9.
People have asked for some further documentation. As they say, this is quite limited, but I did find this page: http://css3please.com/ which is useful for testing various CSS3 features in all browsers.
But testing the rotate feature on this page in IE9 preview caused it to crash fairly spectacularly.
However I have done some independant tests using -ms-transform:rotate() in IE9 in my own test pages, and it is working fine. So my conclusion is that the feature is implemented, but has got some bugs, possibly related to setting it dynamically.
Reviving this old answer because I recently found out about a hack called CSS Sandpaper which is relevant to the question and may make things easier.
The hack implements support for the standard CSS transform for for old versions of IE. So now you can add the following to your CSS:
...and have it work in IE 6/7/8, without having to use the filter syntax. (of course it still uses the filter syntax behind the scenes, but this makes it a lot easier to manage because it's using similar syntax to other browsers)
Thanks Spud! I'm gonna check it out now and get back to you on that. Couldn't believe how little documentation there was about this online.
@ime Vidas and @pagewil - you're right, not much documentation around. I found it documented here: css3please.com, though I should note that this page crashed my copy of IE9 preview when I tried to use the rotate property. However, my own independant tests have confirmed that the property does work, with the vendor prefix as I've quoted in my answer. (given the crash, I guess there are still bugs in it though!)
I really don't get why rotate need a vendor prefix, when they go standard with the rest of CSS3
@Eduardo - possibly because the standard hasn't been finalised yet?