There has been a lot of activity in the CRM forums lately about whether or not Internet Explorer 9 is compatible with Microsoft Dynamics CRM 2011. Here is the current answer as of 3/25/2011.
My answer that can be backed up by other sources that I shall post below: YES - For Release Version, but absolutely not for anything pre-release.
The official MSFT answer: Not in Beta or pre-release, but can't find anything on release version yet.
Sources other than personal use:
I have been using the release version now for a few days and I can say I haven't run into any problems yet with CRM 2011. I have been performing customizations to forms and have even enabled script debugging and stopped on script breakpoints. It just works great.
I hope this helps!