I'm aware that Internet Explorer 9/IE 9/IE9 is not supported according to Confluence 3.5 documentation:
But, see forum thread "IE 9 And confluence 3.5": http://forums.atlassian.com/thread.jspa?threadID=52483&tstart=0
There are a number of people that would like information as to why it isn't supported. They also want to know when support is planned to be added. IE9 apparently has been deployed in customer environments that are using Confluence, and it is reported that when attempting to edit content, it will not save.
I have not reproduced this, but I'm reporting it on behalf of these users.