If you’re a hoster or IT department running Outlook Web Access with Exchange Server 2003, you’ll need to deploy an update to your Front End Exchange servers in order to support Internet Explorer 7.0 (IE7) that ships with Windows Vista.
Here is the overview of this patch:
“In the upcoming Microsoft Windows Vista Beta 2 release, the Dynamic HTML Editing ActiveX control is being removed from the Internet Explorer browser. As a result, there is functionality missing that Microsoft Exchange Outlook Web Access relies on. This update replaces that deprecated functionality on the Microsoft Exchange servers so that Microsoft Exchange Outlook Web Access continues to function smoothly.”
http://www.microsoft.com/downloads/details.aspx?FamilyID=5bc06e8a-08eb-4976-bc68-a03ebe3a2552&DisplayLang=en
You must deploy this patch on the Exchange Server. You can't do anything (from what I can tell) on the Client side to work around this issue. Besides trying a different browser. I was able to use Firefox on Vista Beta 2 and it worked fine.