One way to get around this one is to make sure you're sending to a person in rich text. To do this, add the contact to your Outlook contacts. Open the contact object, and right-click on their email address and choose Outlook Properties
Once in the properties, Simply change the Internet format to Send using Outlook Rich Text Format.
Then just save the contact object.
The next time you send to this person, the calendar request will appear as a calendar request.