I was glad to see that we have a KB article stating this. (Very well written, Nino...) It is a very lo-o-o-ng title: "Exchange 2000 Server and Exchange Server 2003 performance may be affected when desktop search engine software is running on Outlook or other MAPI client computers"
The article does a good job of explaining under what circumstances you will see the issue. This is yet another good reason to run Outlook in cached mode in your organization. Also included is a link to the ExMon tool to identify who are the culprits.
I use the MSN search tool exclusively now and love it. But I also know that I am running in cached mode, so I know that I am not causing any issues with my Exchange servers. Excellent.