A new version of Windows Search was released yesterday. This new version has some improvements related to Exchange Server/Outlook. Check this information that came from the Windows Vista Team Blog (http://www.windowsvistablog.com).
Speaking of accessing Microsoft Exchange Server – if your organization selects not to use Microsoft Office Outlook in cached mode, you can set a Group Policy to index Exchange in online mode. Windows Search 4.0 will then index with minimal impact to the server. Our internal testing of this configuration shows significant decrease in the load on the server and the network as compared to Windows Desktop Search 3.01.
You can also download it from here: http://support.microsoft.com/kb/940157