In a response to a question, Lotus support answered with this:
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"First some background information. When we send an iCalendar MIME message, we include several alternative MIME parts, starting with the lowest common denominator: text/plain, text/html, and text/calendar. Outlook/Exchange prefers text/calendar to be the first MIME part. For interoperability, we place a copy of the iCal in a file attachment and specify that it contains text/calendar. The user can then open the attachment and act on the workflow. The current solution works fine when Outlook is used as a POP3 client.
However when Outlook is used with Exchange, this doesn’t work. When Exchange sees a file attachment with a text/calendar type, it prematurely processes it, converts it into an internal Microsoft OLE scheduling message, and reattaches the new object to the message. Unfortunately, the object is missing some information so the user can not act on it.
The solution is to change the attachment type to application/octet-stream, which is a generic binary object. This will prevent Exchange from processing the attachment and allow the user to manually open it later."
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My question as an Exchange guy is: Where would you make the change to application/octet-stream in Notes?