Concerned about search in Vista and its impact on your Exchange Server? Windows Search 4.0 released earlier this year for Vista and Windows 2008 Server has a number of improvements, one of which is a reduction in the impact on an Exchange Server when the client is operating in online mode...
Have a look at the following articles for more information:
Windows Search 4.0 Released to Web on the Windows Vista blog.
"..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."
Windows Search 4.0 Administrator's Guide on Technet